The mangy dog ​​refers to a person in a relationship between the sexes who knows that the other person does not like them, but yet caters to the other person without any dignity or bottom line. This is not something a man would say if he likes you, it may be a way of rejecting you. Earlier there was a word "spare tire", and the mangy dog ​​was its advanced version.

Of course, this also depends on the context. If the tone is joking, it may mean that they have a crush on each other. You can try to take it further. Contact and interaction. If your tone is harsh, maybe the boy really doesn't like you.