Sehto's fauna in brief consists of the following creatures:
There are some exceptions, but that's the rule of thumb.
The set-up is largely owed to Sehto being in the pokéverse, meaning most macroscopic life is made up of pokémon. As such, there are notably no (non-human) mammals.
Nonetheless, pokémon are not the only animals in Sehto. For one, there are various non-pokémon flavour of fish in the sea (largely owing to the lack of variety in pokémon that are fish-shaped, honestly) as well as plenty non-vertebrate sea-life (coral, starfish, jellyfish, shrimp, you name it). For two, all annoying insects you can conceive also exist in Sehto, most notably whole swarms of mosquitoes just waiting to bite you into an itching mess, and crickets to grant the night its characteristic summer sound.
Linguistic references in speech to real world creatures are absolutely permitted - “It's raining cats and dogs” is not a problem. That works for two reasons:
Narrative is completely fair game.
Sehto pokémon are either Gen I, Gen II or custom-made.
'Custom-made' can under some circumstances (= copious begging and the fond inclination of the gamemaster) also be made to include individual Gen III+ pokémon, albeit only with Gen I/II moves. Gen III+ Abilities can, however, be used if they suit the freeform feel - and they often do! That being said, you're better off freeforming the ability than making a reference to it as a stat, e.g. Magneton may have Levitate but you don't need to make note of that for it to be blatantly obvious that ground moves aren't going to affect it.
Obtainable:
Unobtainable except by sucking up to the GM:
Unobtainable, period:
Creatures in Sehto are placed in a freeform manner. If it strikes you as something someone could run into given the landscape description, it probably lives there.
Read 'no mammals'. Now, ask yourself where that steak you're having comes from. That covers this topic.
In slight more detail: Certain pokémon will be used as food. Tauros and Miltank are obvious, but expect some of the more common bird pokémon to be on the menu, too - and Krabby is probably also delicious. All of that is with the notion in mind that there are plenty fish in the sea (literally) and these presumably form the main source of meats in Sehtoverse. Still, don't expect anyone to flinch at the notion of having a pokémon breakfast. Not even Jagdish Tsukinaka is fazed by that.