Pokemon might have been around for 25 years, but we continue to learn new things about them all the time. Not just by discovering more and finding new favorites (here's looking at you, Lechonk), but also new information about how the relationship between people and Pokemon came to be through Legends: Arceus. What The Pokemon Company has never revealed, and for good reason, is what Pokemon look like on the inside. That had been left to our imaginations, until now.
Biologist Christopher Stoll has been sharing incredibly detailed anatomical diagrams of what certain Pokemon look like on the inside. It might not be canon, but considering Stoll is a professional, and is clearly an incredibly talented artist, it's probably safe to assume the pictures below are what Pokemon's insides would look like if they existed in the real world.
Cyndaquil, Octillery, Donphan, and Bellossom are the four Pokemon Stoll has anatomically broken down. While the images are pretty jarring for anyone who hasn't really considered what a Pokemon looks like if you were to peel back its skin and slice it open (so hopefully almost everyone) they are also very interesting. Octillery's “large complex brain” is similar to that of a real octopus, and of course people have been yucking it up over Cyndaquil's hardened sphincter in the comments.
Speaking of the replies, the general consensus appears to be that seeing the inside of Bellossom will haunt Pokemon fans' nightmares for quite some time. The split image is certainly the one with the starkest contrast as the half of the diagram showing what it looks like on the inside is reminiscent of a clicker from The Last Of Us. Certainly nothing like the “bright petals used for sexual display” you'll find on the outside. You're not likely to find that tidbit in its Pokedex entry.
These aren't the only four Pokemon Stoll has performed an anatomical deep dive on. In fact, the biologist has broken down the inner workings of every single gen one Pokemon and published them all in a book. You can currently buy that book through Stoll's website, but you'll need to be quick. PokeAnatomy is out of print so once they're gone, they're gone. You can also follow him on Instagram to keep up on all his latest work.