What do you really want to say? You want a person? or you want a sheep? If you want to a person who looks after 양, you can say 양치기소녀 or if you want to a female sheep, then 암양

Girl would be "Yeoja(여자)" but depends on who your saying SheepGirl too. A teen girl would use 소녀 but a normal girl would be 여자. Most people use 여자 though. So it would most likely be "YangYeoja (양여자)" and not YangSonyeo (양소녀) but i'm not positive because it's not my first language and i'm trying to improve it. I hope i helped :PP

Sheepgirl = 양소녀