noun
small horse; pony
Refers to a horse of small stature, including ponies. Not necessarily a young horse.
あの牧場には子馬が何頭もいる。
There are several small horses at that farm.
noun
foal; colt; filly
Specifically a young horse, typically under one year old. Colt is male, filly is female.
春に生まれた子馬が元気に走り回っている。
The foal born in spring is running around energetically.
Compound of 子 (ko, 'child') and 馬 (uma, 'horse'), with rendaku voicing of the second element.