That's my favourite mistake in English...I do it almost every time.
We have...three gender system, I think all Slav's countries have the same system, but I'm not sure.
Masculine, feminine and neuter.
I'm not an expert in linguistic field...but easiest way to explain is...that if an noun have an -a at the end...than it's feminine gender.
Squirrel on Serbian is veverica (veveritsa),so it's she
without looking under the tail