What could this lump be on my mum's dog?

My mums dog shihtzu has a lump next to the top of his tail it is about the size of possibly a g maybe a little smaller and feels ball like. It looks like it may have been irritating him as there is a scab but doesnt look fresh. The lump looks a little dark (poss could be dark as part of scab not sure cant tell). What could it be i only noticed it tonight?. thanks