Magic animals?

I already have some magic animals, including the sheep. However, I’m in winter Y2 and wanted to have one of the rainbow sheep for the rainbow wool. I have the sheep the fertility potion, visited her and gave her her favorite food everyday for the 7 days but she still birthed a regular sheep? What did I do wrong? I already have 1 magic sheep (the purple/pink one) should I give the potion to that one?