Please take him/her to the vet as soon as possible.
My late cat had one when she was 10. It was sarcoma and we caught it on time. The vet was able to extract it totally and she was ok for another 5 years!
How large is it? and is it kind of like an outward growth or an inward growth?
It could be something as normal as a skin tag or it could be something more.. Yes, I agree a vet visit is in order... or at least a call to the Vet....
Agree with madzoya. My cat had one on her hip that was a sarcoma and was removed and never came back because we caught it early. It is only treatable early.
it ended up being a skin problem i got medicine to spray into his mouth for the problem but its a real adventure holding him and trying to get his mouth open the first time wasent to bad but hes smart and as soon as he sees the medicine bottle he runs