YOU SAID:

While i was riding a cow my horse got really angry so he killed the cow

INTO JAPANESE

私が牛に乗っている間、私の馬は本当に怒ったので、彼は牛を殺しました

BACK INTO ENGLISH

He killed the cow because my horse was really angry while I was riding the cow

INTO JAPANESE

私が牛に乗っている間、私の馬が本当に怒っていたので、彼は牛を殺しました

BACK INTO ENGLISH

He killed the cow because my horse was really angry while I was riding the cow