Hello Dear Aspirant!!
Upsc offers optional in various subjects including Zoology. Selecting a right optional is very important to secure a good rank in this exam. The optional you choose must be of your interest because you will have to know everything about that particular subject. As you have mentioned you are doing your graduation in Zoology you will have an upper edge in that subject but take it as a optional only if you have genuine interest in the subject. You can take optional in any subject regardless of your subject of graduation.
I hope this helps you!