Magcargo
: The Slugma family has an interesting presence here. While most other games have their territory near volcanic areas (even in Kanto, geographically they're not too far away from Cinnabar Island), in Kalos they're hanging out on Route 13, the badlands. It wouldn't seem there would be that much volcanic activity here (or really anywhere in Kalos, at least that would be suitable for the Slugma family; maybe Terminus Cave?), though if you remember what's also here there's a good reason for them to be here: The Kalos Power Plant. One of the ways the Kalos Power Plant draws power is through geothermal energy. Underneath the badlands there must apparently be some kind of hotspot that is being tapped into, and as a result the Slugma family is here representing the source of the heat:
MAGMA. While some probably did come to the surface on their own before the Power Plant existed, the hotspot now being tapped into would give them all a direct passage from deep underground to the surface above.
Magcargo is primarily here to get Sunny Day set up (and maybe burn anyone who thinks making physical contact with living lava is a good idea). It it has a second turn, Stealth Rock. And if it somehow survives to a third than it can try either releasing an Overheat or provide support with a last second Yawn. Still you don't want it out too long, even if Heat Rock gives Sunny Day a couple of more turns.
Torkoal
: Speaking of Terminus Cave, as you could tell from the old cart tracks and equipment, it was an old mine; a
COAL mine. And just outside, on Route 18, there's the Pokemon ever searching for coals to keep their furnace shell ablaze: Torkoal. Yeah, while many relate the Slugma family to Torkoal (being they were right next to each other in the Hoenn Dex, I think some people even thought Torkoal was a new evo for Magcargo), and them sharing similar environments, in actuality they generate heat from two different sources. As a Fire-type Torkoal can produce some flame, but it needs to burn coals in its shell to really reach scorching temperatures.
While I list Torkoal second it's actually the back-up Sun Setter using its Drought Ability. With Magcargo having the Heat Rock, and Torkoal being a bit more capable, while setting up sun is its goal it can dish out damage too. Just like the coals burning themselves out to release their energy, Life Orb may make it not last that much longer but leave an impression. Overheat for that STAB overkill, Solar Beam to take advantage of the sun for super effective coverage against all of its weaknesses, and finally Earthquake and Gyro Ball for some strong physical options after it does one or two Overheats.
Heatmor
: We got heat & coal, let's talk about that final flammable resource we extract:
NATURAL GAS. Whether in liquid or gas form, most organisms that aren't humans would likely avoid the stuff as it has the tendency to explode or catch on fire (I'm sure there's some extremophile bacteria which found a way to feed off it, but that's an very extreme & specialized case). However this is the Pokemon world, so if it exists that means there's a Pokemon which either embodies or utilizes it. In a sort of arms race against their natural prey Durant, Heatmor have evolved to take in air from their tail and breath it out as hot flames (to melt through Durant's steel armor); and while it doesn't say it needs natural gas to do this, I don't think it would necessarily hurt it.
With the niche Ability Gluttony, I gave it a Petaya Berry to eat early so get a boost in Special Attack. Not only does this give Fire Blast and Solar Beam more power, but it also lets Heatmor use Belch giving it a powerful Special Poison-type Move. Finally, with it having no item, Thief may by a Physical Move but it serves Heatmor quite well, if not giving it a useful item then by taking away one from the opponent; either way a win-win.
Chandelure
: Now we start moving away from natural resources humans could use to biological mechanisms. And for Chandelure we all know what its source of heat is:
SOULS (& LIFE FORCE). It absorb life force and eventually the entire soul to use as fuel for its ghostly flames, and really that's as much detail as we get; Maybe its better that way.
If there's a non-Mega ace of the team it would be Chandelure. Its a powerhouse that increases its Special stats with Calm Mind, fires away with Fire Blast & Shadow Ball, and heals what it can from the damage it does with Shell Bell. And for a little fun, it has Curse as an option.
Drapion
: If we're talking about biological mechanisms and heat generating, we have to talk about one commonly used by organisms as a defense mechanism (or a tool to hunt):
CHEMICALS. And among the kingdoms best known for using biological chemicals its the bugs. Some are able to spray a concoction of liquids which when combine creates an intense heat and maybe even a bang. Though the far more common method is via venom, stinging a target and releasing toxins which, if it doesn't actually cause heat, makes it feel like their blood is on fire as it damages and causes necrosis from within. Such as from a sting of a scorpion, though Drapion has more than just a poison tail.
Drapion seems to have some bombardier beetle in it as its able to light its mouth on fire to use Fire Fang, but I digress. Cross Poison and Night Slash are the expected STABs, but there's more to them when you consider Drapion has the Sniper Ability and holding a Scope Lens, giving it the best shot to score Critical Hits. The final surprise comes with its last Move, Sandstorm, which isn't really for it but the final Heat Bringer...
(Mega) Garchomp
: Finally some biological mechanisms is just an adaptation that makes it easier to live in its habitat, even if that adaptation requires generating a lot of heat. For a Dragon-type, the Gible family are sure fond of cold and damp locations. You'd think a Dragon double weak to Ice would stay away from the stuff, but the Gible family have learned that through their digging they can generate high temperatures through sheer
POWER. Maybe its from the kinetic energy caused by the friction of digging or from their dragon heritage they're calling forth; eitherway this comes to a head with Mega Garchomp as the Mega Energy has caused so much heat its wings have melted into red hot scythes!
Just all power, especially if Drapion can set up that Sandstorm. Fire Fang may not be that strong but makes for a decent coverage against bulky Grass- and Bug-types; Earthquake and Outrage are the STABs, 'nuff said; and finally Iron Head deals with any Ice- or Fairy-types that think they can bring an end to this heat.