It really all boils down to what type of game it is. For example, Xanje is focused on collecting and capturing pets, so new pets are generally released quite often.
Then you have other pet sites like Neopets, Marapets, and Subeta that aren’t focused on collecting or capturing pets, so introducing new pets isn’t as common, unless they’re tied into some kind of lore or storyline.
Adoptable pet sites also release new pets more frequently than games like Neopets and Marapets, since you collect an egg and raise it, or users are able to customize and launch their own pets into the game. Every game has different mechanics and loops, so introducing new pets should be based on how those systems work. Adding rare pets gives users something to work toward, but I don't think it makes sense to release new pets constantly without a solid reason for them.
Users tend to grow attached to their pets, and overloading a game with too many can feel overwhelming. However, breeding mechanics give pet sites a great way to introduce new pets, but those are usually only available through breeding. If you don’t breed or get them from another user, you can’t obtain them. That’s part of the core game, though, so it naturally adds to the total pet count.
Breeding, collecting, and capturing systems create a wider game loop than just “Adopt a pet” or “Create a pet.” So, those kinds of games will naturally have more pets and that’s a good thing. Having a variety means you don’t need to keep adding new ones all the time. Releasing new pets during holidays, special events, or monthly updates works well for games with breeding features.
Neopets has been successful for many years with just its core pet lineup and they only recently introduced a new pet. That tells you something: more pets aren’t always necessary. What really matters are the features built around them.
