Hoses can run very fast.为什么要加s？

Horses can run very fast. 妈能跑得很快。
horse是单数可数名词，不能单独使用，要么其复数，要么在前加限制词，如：
A horse can run very fast.
The horse can run very fast.
My horse can run very fast.

说的是同类，泛指。