Ah, thanks for bringing that up. I didn't think that they might have just meant the custom adopts you find in the Oekaki boards ;;;;
But yeah, if that's what you meant, there is that.
There are also special user-made species in the oekaki boards, like "open species" and "closed species". An open species is one that anyone can color in and keep, and a closed species is one where that species has a staff dedicated to moderating that species and creating designs for it, then giving them to users through different means such as competitions of some sort.
If that sounds fun I definitely recommend checking it out!
Kalopsian already linked to the user-made adoptables. Open species are usually found in the
editable oekaki forums, while
closed species can be found in the adoptable oekaki forums.
Sorry for any confusion!