It looks like a seal jaw to me. You don't say where it was found. In the UK the are two common types of seal; grey and common. Grey seal have jaw lengths about 20cm but they look more robust than this (although I am comparing with a male) and the front three molars teeth have a single point and the back two molars two or three points. Common seal have jaw lengths around14cm and all the molars have multiple points. I think it is a young grey seal, possibly a female.