"They are big animals."
Translation:Jsou to velká zvířata.
Well, it is better to take it this way:
There is a very common sentence type "To je...", "Je to...", "To jsou...", "Jsou to...".
It is VERY common and there are many people asking it in many places of this forum. The "To" there stands for any indeterminate thing or things of any grammatical gender. Just remeber that.