So, I've recently been reading about the lore behind the creatures of Subeta, and I've come to realize that a LOT of the pets that are available on the site haven't been written down what so ever. Is this a mistake or was this on purpose?..

It's a work in progress :)

Yeah, it's something they've been working on over the past couple years or so. They usually release the lifelike dolls for a pet at the same time as they add the Subetapedia entry.

Since there are so many pet species to come up with info for, it could take a while still before they're all added in there. But it should happen eventually.