3 year old neutered male. He used to be a stray, but he is now indoor only since we adopted him about a year ago.

Nearly the whole time we’ve had him he’s had this weird bump on his left shoulder. It’s hard and solid, but it doesn’t seem to bother him. He lets us move the fur away and inspect it (if he doesn’t like something he’s not shy to let us know, so that’s a notable detail). I don’t think it has changed shape or size much since we first noticed it. We asked our vet to take a look when we took him in for his last check-up, but he was in a spicy mood after she gave him his shot and he did not want her near him for any longer lmao

Does anyone have any ideas on what it could be? The closest looking possibility I’ve been able to find so far is that it resembles a keratin plug, but I wanted to get some other opinions.