sheep为什么没有复数形式

　　sheep，n.羊;绵羊；复数： sheep

　　例句：

　　The minister preached a sermon on the parable of the lost sheep.

　　牧师讲道时用了亡羊的比喻。

　　The first task was to fence the wood to exclude sheep

　　第一项任务就是把树林围起来不让羊进去。

　　I'd kinda like to have a sheep farm in New Mexico

　　我有点儿想在新墨西哥州开一个牧羊场。

　　Sheep's milk is produced in much the same way as goat's milk.

　　绵羊奶和山羊奶的生产过程差不多。

　　Dogs were allowed to roam free and 48 sheep were killed.

　　没有对狗加以约束而让它们四处乱跑，结果有48只羊被咬死。