the three diamond dogs from the original series...
(and before some ask - yes, the name is a David Bowie reference)
and they end up as human too.
Oddly, Spike becomes a dog.
It is never explained if specific races become human while others become animals.
I wouldn't exclude that the sea ponies just become humans like regular ponies do.
I even think that one official EG comic has an human Gilda in the background in on scene.