There's been quite a few Pokemon I've picked up towards the latter half of any given game, usually right after the 6th or 7th gym or its equivalent, and usually as the result of me going "These 4 or 5 team members are already really good buuut I
might as well pick something else up, just to round out the team". Here's just a few of the best ones
Vileplume was something I decided to add onto my Omega Ruby team after finding a Gloom in the Safari Zone, both because I'd had success with the line in randomized nuzlockes in the past and because I just figured it'd be the best way to round out my 5 mon team of Swampert/Swellow/Electrode/Machoke/Linoone. It did
not disappoint, not only helping me with Wallace but also putting in quite a bit of work in the E4. But where it
really shined was when I went legendary hunting. Decent bulk + Sleep Powder+ a STAB combo that is great at carefully whittling down most legends while also doing enough damage to get them into catching range quickly was a godsend. I have nothing but respect for the line after the performance it put on
Ultra Moon was one of the rare cases where I had
two lategame additions, both of them actually coming at the same time, which was right after the Mimikyu trial. By that point, I'd actually been looking for good options for the last 2 slots of my team for a while, and had gone through a few duds (Alolan-Duggy, Larvitar, even Pineco) and was beginning to get worried that I'd only have 4 Pokemon by endgame (those being Primarina, Hawlucha, Malamar, and Ampharos. Honestly I wanted 6 mons just for the sake of having a full team because those 4 probably could've solo'd the rest). It was then that I remembered that not only could I find Trapinch in the desert, but that I could find Haunter in the Abandoned Mall. Seeing as I was really wanting for good Ground and Poison types at the time, I quickly hunted them down and caught them up, even getting Haunter to become Gengar immediately thanks to me having 2 systems. Almost immediately after reaching their final forms, they really began to shine, blowing through most routes and even some bosses, with Flygon destroying Molayne and Olivia and Gengar ripping Acerola apart.
Magneton was brought onto my Leafgreen Nuzlocke team as the result of a tragedy. See, after beating Blaine, I'd decided to do the Sevii Island quest since it unlocked some new trainers, a great grinding spot, and a couple of new encounters, all of which are excellent for nuzlockes. However, when I was doing the biker fights on Three Island, I'd decided to try and have my Butterfree Harvest solo all of them and she did...until the last Weezing, which tanked a Psychic and used Explosion, killing itself...and taking Harvest down with it. Harvest had been with me the whole game, so I was crushed at losing it, and I also hadn't planned on having a slot open. But after plowing through the rest of the game and getting to the E4, I realized something: my team wasn't ready. It could take on most of it no problem, but Lance and Lorelei were very intimidating owing to their strong Flying, Water, and Ice types that I had no real answer for. Figuring that I might as well fill that empty slot with
something that could give me a chance vs them, I decided to go down to the Power Plant and get my free Electric, which turned out to be Bastet the Magnemite. After grinding it up to speed, I decided to try it vs the E4 and see what happened---and it didn't disappoint. It not only
crushed Lorelei, soloing her team with minimal damage, but also put in work vs Lance's Gyarados as well as the Rival's Pidgeot (I used Green Day the Venusaur to beat his Gyarados which, while risky, had such a cathartic payoff). Honestly I'd say it was the only reason I was able to beat the E4 without any casualties
Following the trend of "Last minute additions to Nuzlocke teams specifically for the E4", Painting the Electrode and Department the Rhydon (The naming theme was random nouns) were added to the team following both the death of Poet the Beautifly at Tate and Liza and me getting to the E4 and realizing that a team of 4 mons just wasn't going to cut it. So using my Safari Zone and New Mauville encounters, I picked up these two, grinded them up to speed, and threw them at the E4, and like Bastet before them, they didn't disappoint, with Department bulldozing through Phoebe (the fight I was the most worried about), and Painting putting in serious work vs Glacia though sadly dying to her Walrein, though not before giving my Blaziken the edge it needed to win. For being literal last minute additions, I really liked these two and I'm glad I got to use them. Also I've apparently developed an affection for using Gen 1 Electric types that come after you get Surf...huh
Abomasnow, unlike the other Pokemon here, was something I at least conceptually planned to have on my Platinum team the moment I could find it, since I needed some sort of Ice type for Cynthia's Garchomp. I was hoping to use something else, but given my self imposed rules of "Sinnoh native lines only" and "no type overlap", it just so happened that Abomasnow was the best choice I really had. To be frank, it basically saw no use until Cynthia herself (though it did kill a couple members of Bertha's team), but when it came time to take on the Champion, it performed well, revenging Garchomp after it muscled through my Empoleon and even manging to defeat both Milotic
and Roserade (The AI switched to it predicting a Grass move and I called it perfectly) and chipping Togekiss. I had to baby it severely to get it to this point but the payoff was more than worth it
The final one I want to mention is probably the least spectacular of the bunch and that's Grapploct. As with Vileplume, Flygon, and Gengar before it, I had just beaten the 6th gym/gym equivalent and was still rocking an empty party slot or two. This time around, I was just kind of winging it in regards to party comp and had passed up some otherwise solid additions that I either wasn't feeling at the time (Falinks, which in hindsight I regret not using, Impidimp), couldn't find (Sinistea), or couldn't figure out how to evolve and didn't really care to put in the time ot learn (Applin>Flapple). I was also nervous about dealing with Hop's Snorlax but moreso Raihan's Duraladon. As luck would have it, I managed to encounter the static level 50 Grapploct and decided to add it to the team. It was perfectly fine, taking care of all the Normals and Steels as it should (and also giving me the most hilarious Chairman Rose matchup with my entire team being good or at least neutral) vs Steel. It also did really well vs the final Marnie fight though frankly anything could have. Honestly still kinda wish I'd used any of the other mons I listed just because I like them more but Grapploct did its job pretty well and rounded out the team perfectly fine.
There's a few others that fit this category, but I just wanted to talk about the most notable/good ones.