I know there's another thread but it's looking cluttered so I'll be posting here instead. It's going to be a long time before this thread reaches the very end of the catalog anyway (if ever). Feel free to cross-post to the other thread.

Found a couple of pony ROM hacks on this ROMhacking.net
Pony Poki Panic: https://www.romhacking.net/hacks/791/
My Little Pony: Dr. Discord's Conquest: https://www.romhacking.net/hacks/1731/
There could be more. I merely searched "pony".