Most of my 'wild' species/creatures are copied within variation from various myths and common fantasy archetypes. Occasionally a variant, such as a clan of mixed oni and lycans/wolf-shifter bloodlines caused by a small clan of each being trapped together in a valley for sufficient generations to make intermarriage common.
I am also running with a variant of dungeon core, and there I simply play withing what feels appropriate for the dungeon and floor/zone. The first dungeon has rabbits, and they have bats, so use the information from bats to evolve some of the dire rabbits into rabbats. Both the 'normal' dire rabbits and the rabbats get various elemental attacks like breath weapons.
The library zone has bookwyrms (tiny origami dragon constructs, complete with paper-fletchette breath weapons), biting words (yes, books that bite), and bunbrarians: Powered up rabbit-people with giant hammers who show up to enforce silence on you if you make too much noise.
Shade Tails: magical black squirrels who can use their tails to lob shadow/void energy attacks.
Hex wolves: Wolves that have six tentacles growing from their shoulders and sides that inflict random hexes if they hit you.
They end up with a population of pixies, somewhat by accident, and later on an ocean-themed zone is gifted with mer-pixies, who are always
delighted to be 'helpful'. To their credit, they are actually trying to be helpful. But, well, pixies.
One of the bosses of this zone is a giant jellyfish with electric powers in top of its toxins, and the electric powers are the basis for some psychic powers as well.
For a different dungeon that is a bit more focused on direct combat trials, it has gorrilla-taurs, basically centaurs with Clydesdale+ sized horse bodies with gorillas instead of humans for the upper torso. Also, they are just as good with bows, and the bows are upsized to match in power and size. And some hippo-boars that are upscaled and have reinforced boar-bristles on top of hippo hide and supernatural resilience.
Earlier in the same dungeon, there are some gorillas who are warrior monks, complete with ki-based powers.
So yeah, I just run with fun ideas when it comes to dungeon creatures, because there is a person who is literally designing these and presumably having fun.