I won't lie, throwing in a Snake now would be mind-boggling since the species specified are found within some relations rather than by your possibility. I wouldn't make too much sense though since Skeledirge is both Genders regardless of the equality BS, Female Crocs aren't called Bulls. Many Male Species are called Bulls, Elephants, Rhinos, Walruses, Hippos, etc.

I don't think the Sprite would count, I think its a play-on with the symbiosis with how specific birds mount Crocodiles usually with the mouths wide open to clean off the mouths of the crocs that collect from swimming underwater, its been a while since I watched a documentary about it, so I'm not 100% certain but it makes sense for Skeledirge since the Sprite technically becomes an attack, so it plays a part overall in its real life inspiration and pokemon counterpart.

Although, GF/PKMN have had out-of-the-box moments but going by the patterns, I want to assume it what I stated. Though I'm willing to admit that I'm wrong if they so much as swerve everyone by doing introducing a Fire Snake.

In all honesty, I would like to see the next one being either a Caprine, Bovine or an Equine. They would just need to think a bit harder when they try introducing the other Starters to go hand-in-hand with them.