@Mr100dragon100
Honestly, the biggest bit of advice I can give you is two things. One is just don’t overthink it. Two is go all the way with whatever you come up with. Either take a creature, give its features to a pony, and then come up with some lore, or come up with a concept and find creature features to justify it :D
I’ll do one right now. How about a creature that feeds off of electricity, and bites into electrical wires to feed? So, to me, that’d have to be a creature that can either jump, climb, or fly, and would have to be smaller than a normal pony, so right off the bat I think insect. So spider ponies, ant ponies, fly ponies, and most bug ponies have been done, so what’s a biting insect that hasn’t been done yet? So I think, how about a tick? So now you have a tick pony, except instead of being gross and biting other creatures to suck blood, it is harmless to other creatures and latches onto electrical wires to feed. And, now we have a design: it’s butt is like a light bulb that grows bigger and glows brighter the more electricity it feeds on :D
So, you see what I did: I picked a core concept, picked features from the animal kingdom that reflect that behavior, and came up with something new. It’s harmless, it’s rooted in reality but fantastic and magical too, and its behavior and physical features logically line up. And if ticks are too gross, there’s no end of biting insects you could use instead :D