It's very hard to say from the photo, but it could be boar glue. This is a typical place to find it. Is it rock hard and stuck to the hair/skin, white to cream in colour? If so, that would be my first thought. It's nothing to worry about, just something you get with some boars when they clean themselves. If it doesn't sound like this, get a vet to check it out, especially if it changes or grows.