Seriously, when I think of magic creatures, my brain doesn't even go to the mainline fantasy epics first. Everyone will shout about 'Harry Potter' and 'The Hobbit', which are valid, but the most interesting beast dynamics happen in books that treat creatures like a core magic system component. Like, in Naomi Novik's 'Uprooted' and 'Spinning Silver', the magical entities—the Wood and the Staryk king—aren't just pets or mounts; they're sentient, environmental forces that the magic directly interacts with and draws from.
That's what makes a magical creature 'powerful' for me. It's not about how big its fire breath is, but how its existence warps the rules of the world. There's this weird little book called 'The Raven Tower' by Ann Leckie that flips it entirely—the 'creature' is a sentient stone that is a god, and its power is geologic, slow, and absolute. More traditional but done incredibly well is 'The Priory of the Orange Tree' with its dragon lore that's deeply tied to political and religious structures.
For a real deep dive into a symbiotic human-creature magic system, you can't beat the 'Books of the Raksura' by Martha Wells. No humans at all, just shifting, flying court societies with their own biologies and magics. The creatures are the people, and their power is in their social structures and adaptations.