Level 1
Impossible to beat the game: The game doesn't save after the final boss, so, even after you do it, you reload your game in a pre-final boss state. Cool.
Eye fish: The game has a fishing minigame, my favorite fishing minigame ever. The hardest fish to catch have an icon with an eye on it while you're trying to catch them. Besides being very rare, these fish are also legitimately hard to catch in the minigame lol - I am an adult and still sometimes fail to catch them. I actually had to change my form because I was focusing so hard I forgot to breathe and got chest tightness lol. You never have to catch an eye fish aside from one optional quest*, so I bet many kids never caught one.
Rosemary / Orchid: My favorite part of the game is the collect-athon aspect. You collect a lot of fruits, vegetables, insects, fish, and flowers scattered across the world for full completion, since you need one of each type for it. There's a lion in front of the Rosemary flower you have to bait out, and he doesn't play around - when I tried to get the Rosemary as a kid, he trapped me in a corner and attacked me to death. It was pretty scary!! The Orchid requires you to jump in a poison bog. So lol.
IC / CC Secret Tunnels: There are two secret places in two areas, Inferno Cave and Crystal Caverns, that you can only access with a key item you get later, requiring you to come back later.
Level 2
Version Differences: OK, this might come as a surprise but... in the Dogz version there are dogs and you are a dog, and in the Catz version there are cats and you are a cat. And you can bark as a dog, but you screech instead as a cat. More interestingly from stuff like that, though, there are some minor gameplay differences. For one story quest, you get an Apple in Dogz and Cherries in Catz. Little unnecessary things like that make me love this game. An optional area in Catz has a fishing quest, while it has an insect quest in Dogz. Makes sense... cats do like fish.
Minigame Accessories: A fun little draw of the game is all the accessories you can put on your animal and a lot of the other characters, too. Unrelatedly?, another part of 100% completion is returning animals that escaped from the Zoo, and you make them go there by beating them in a minigame. After you do that, you can see the animal in the Zoo and replay the minigame if you want. Seems like there's no explicit point besides playing the game if you want to, but every 5th game you unlock an accessory.
Longer Stuns: A big part of the game is that you can never defeat enemies, just get away from them. You can stun them for a while, though, if you bark / screech near them or hit them with a rock. It's only a handful of seconds if you bark quickly, but a whole minute if you hold the button a while. Really convenient.
Chicken Pond: One of the minigames you play with a Zoo animal is a chicken race, where you try and run as far across a pier as you can without overshooting and falling into a lake. The pier exists in the overworld, as does the lake, and you can swim in it if you want.
Instafreeze/Burn: One area in the game is really hot, and one is really cold. To get in the areas, you have to wear protective clothing, and even then your temperature changes over time, damaging you if you stay around too long without cooling down / heating up. The game won't let you in if you refuse to wear the protective clothing, which makes sense..... but you can take it off while already inside the area for some reason (??). Normally, when you take damage from exposure, it's 1/2 heart per 1/2 second. With a maximum of 10 hearts, that's scary but reactable. If you take off the protective clothing in the area, though, you immediately start taking 1/2 heart damage per frame. Good luck with that one.
Level 3
Eye by Size. See "Eye fish" above. You'd think the largest fish would get the eye, because they'd be harder to catch. Or maybe the rule is that certain types of fish are especially strong fighters and get the eye. Which is it? Both... kind of. Some species always get the eye, but some only do when they're relatively large enough. So you get situations like, a Swordfish will never have the eye (300cm - 400cm), while a Ray can (100cm - 200cm), but the Ray will only have the eye if it's 200cm exactly. Weird stuff.
Variable Damage: When an enemy hits you, they do a set amount of damage. Every enemy except two... the one light purple Cobra and the one red Cobra in Crystal Caverns, which can either do 1 ½ or 2 hearts (light purple) / 1 ½ or 2 ½ hearts (red). The game never tells you, so you have to get hit multiple times and be really observant to know.
Item Setups: This one's a doozy. Okay. Remember the collect-athon items? Lots of items are "possibly available" whenever you enter an area, but not all of them will load in each time. They have a bunch of different rules determining one which ones will enter. This is most discrete for fish and bugs, which have 4 (fish) or 2-4 (bugs) different possible configurations of species and rarities/locations ("setups"), one is loaded every time you enter. This can be especially rough for fish... the game gives you a hint where to get each item, but some fish are setup reliant. It tells you to get one fish, the Hairtail, at a place where it's only in one setup. Translation: every time you enter the area, there is a 75% chance the Hairtail will never be catchable, no matter how many times you try. And there's another place to catch the Hairtail where this isn't a problem. And the game doesn't tell you about it.
Crouch Canceling: One way you escape enemies is by hiding in tall patches of grass, crouching. You can cancel your crouch by barking, or a couple other things. The name is tongue-in-cheek: canceling your crouch makes you visible and gives you no advantage itself, so, unless you are not visible another way, you're exposing yourself to enemies for no reason.
Sniffwalking: You sniff (or use your whiskers if you're a cat) to find hidden items. Sniffing while moving has a couple odd properties. Most importantly, it stops a run from accidentally getting inputted, which makes you avoid waking up sleeping enemies.
Homing Throws: If you throw a rock to stun an enemy, but your aim is slightly off, the game will slightly autocorrect for you by making your dog turn to face the right way as you throw. Clever. This homing is especially prominent for the Dodge Ball minigame with the Monkey from the Zoo. However, he gets the homing, too. No cheating!
Level 4
Rockless Rosemary: The primary way to get the Rosemary (see above), the way the game tells you to, is to throw a rock at the lion to bait him out of the way. You can just walk up beside him, bark, and sneak around him. Way easier and less risky.
Egrets Aren't Real: Flying enemies are especially scary in this game, as they fly way above the camera, swoop down to attack without any warning, and are basically impossible to counterattack. They legitimately scared me as a kid in this game and still do lol. One of the flying enemies are Egrets, which appear four times in the game. Two guard stretches of water in Lappy Lake, but you never need to cross into their territory to access any of the islands, so you don't need to mess with them at all. One guards an island in Whisker Woods, but the island has no items in it, so you never need to go there and mess with it. One only appears temporarily for a quest and is entirely passive. So, in short, Egrets are these really dangerous enemies that... you never have any reason or need to interact with all, once you know they don't guard anything.
Mock Charges: Some enemies act like they're going to charge you and attack, but they stop and attack way early. The Gorilla does this, and I wonder if it's a sneaky reference to how Gorillas are pretty chill and bluff-intensive in real life. Or maybe I'm overthinking this.
Destroyed Tail Heights: One area leads into another, Tail Heights into Sky Heights, with a large gate. If you stand at the side of the gate from the Sky Heights side and turn the camera around, you see a blank cliff drop into the sea, instead of the rocky highlands path you came in through. Minor graphical oversight? No, you destroyed the entire area by looking back. Sorry.
MI/WW Secret Spot: Two areas in the game, Monolith Isle and Whisker Woods, have out of the way areas you never have any reason to go to, but they're there! You can run along the edge of Monolith Isle and hang out in a weird spot beneath a cliff, and you can swim in an out of the way branch of the river in Whisker Woods. Cute little not-striclty-necessary areas like this make the game worlds feel more organic and alive.
Level 5
Freeze faster than Burn: In the extreme temperature places, from neutral temperature, you take like 70 seconds to overheat and like 45 seconds to freeze. The latter area is a bit later in the game, so I guess it makes sense there's a bit more challenge, but it's something I never noticed or would've thought to check as a kid, until I decided to check it because I was curious.
Snake Patrol: Enemies have idle behaviors when they aren't noticing and chasing after you. A lot stay still, some stay still or move around a bit, some turn about. A lot of snake enemies stay still most of the time, but rarely conduct an elaborate little patrol loop of their territory. It's a really sweet small little detail. In enclosed areas, they can get stuck against walls for a long time doing this, which is a really funny little accident.
Fake Safe Enemies: Enemies have little icons to tell you whether they've noticed you. The Safe green icon means they don't see you... unless they're a Lion or Komodo Dragon, where it might mean they've noticed you but are seeing whether you'll get out of their way. If you don't, they'll quickly chase after you and attack, which is a pretty mean trick.
Level 6
Scrapped Bubble Cave Exit: To access the Bubble Cave area, you have to talk to a character in on Monolith Isle, who will move a rock blocking the way in a cutscene. To leave the cave, you have to talk to them again, starting another cutscene. You can walk past the character towards Monolith Isle to try and leave, but an invisible wall will block your way. If you walk up to the invisible wall, turn the camera way, and turn it back towards the wall, text for Monolith Isle will pop up, which is how the game tells you that you're walking towards a new area. You still can't cross the wall, but maybe you were able to at one point in development, if this text is present but suppressed.
Ice Piers: Normally, after you catch a fish in fishing, you can immediately press the fishing button again to catch another fish. However, in the extreme cold area, you have to walk a bit closer to the water every time before casting the rod again. Almost like they want to make you take a bit more time to make it a bit easier for you to freeze to death.
Unsafe Zoo: Some of the Zoo animals look identical to animals who are enemies. Like the Zoo Vulture looks identical to the Vulture enemies. Makes sense... but. Normally when you run into tall grass, you crouch to evade enemies, but that doesn't apply in the town areas, because there are no enemies and thus no need to hide. There's no enemies in the Zoo either, but, just like in areas with enemies, going into grass automatically makes you crouch, as if there are enemies you can't see. Are the Zoo animals going to rebel and start attacking you? No, but imagine if.
Level 7
Sleep Mechanics: I still have no idea what makes enemies go to sleep, and when, and why some seem to go to sleep more often than others. The game has a time feature where the sky will change between day and night, and this has no effect on gameplay-except for maybe making enemies sleep at different times? They seem to sleep more often at night, which may explain why night is shorter than day in the game, but they can also sleep during the idea. I have no real idea here.
Loaded Insect Setups: Through preliminary research, I think that the metaphorical die to decide which insect setups get loaded is, uh, loaded. This could be sneaky way-on top of another sneaky way in setups-to make some insects rarer, maybe. I imagine there could have been another way than these two layers of totally unexplained mechanics, the second of which seems to only apply to insects, to make some rarer, but who knows, this game is just like that.
Secret Extra Thing I Just Found Out Today
One of the obstacles in Sky Heights is a poison bog you have to walk a specific path through, using your sniffing to find it, to avoid getting poisoned. Or you can just run along the right wall, barely avoiding the bog by clinging to the shore. Oops!