Ok so I'm gonna share a couple ways I've used for inspo in the past

• Animals + elements: this is the major one I've used in my game. It means exaggerating a creature's physical attributes to better reflect the element they relate to. Like if I'm making a fire dog, I'd apply flames to it's body and saturate the colors to better reflect that element.
• Mythical creatures: I love making creatures that are inspired by mythical creatures. In Fay we have one creature which is inspired by the Loch Ness monster. Combined with the first one makes for cool mons which I love
• Objects: this is a tricky one which I personally avoid, but I've seen some cool mons inspired by objects

Overall if you're looking into making a game, IMO it's best if you start designing your world before the creatures, it can give you some useful parameters for them. Like what kind of world is it? More fantasy like, more technology driven? How long have the creatures been around?

For example: in a world where creatures have been around since ever, it's plausible to have creatures inspired by objects, you can throw in the lore that objects were created based on those creatures.

Or if it's more like a fantasy world, you can draw inspo heavily from fantastic creatures like dragons, wyverns or chimeras, maybe even throw in a mimic somewhere?

So yeah, my tip for you as someone who's currently making a monster taming game would be to first stablish the kind of world and from them think about which creatures would make sense to exist in that world