YOU SAID:

I went to the shop and bought a packet of milk for my pet rhinocerous

INTO JAPANESE

私は店に行き、私のペットのサイのために牛乳を買った

BACK INTO ENGLISH

I went to the store and bought milk for my pet rhino

INTO JAPANESE

私は店に行き、私のペットサイのために牛乳を買いました

BACK INTO ENGLISH

I went to the store and bought milk for my pet rhino