Nupe can't see the pics.
However, since the vet's looked at the horse, it's been "down the pike" for tests and meds. the bump got bigger and is now "hard as rock", my best guess is the horse got bit by something and is having a really bad allergic reaction.
You don't say where you live, but any kind of poison spider (like a black widow) could do something like that. Scorpions might also do that.
I know for fact there Black Widows behind the kickboards in my barn because I've seen them. They do their best to stay out of the horses' way but I'm sure if one of my horses accidentally rolled over on a Black Widow and it got a chance to bite them in retaliation, it would.
It sounds as if the lump may end up having to be lanced by the vet. The chances of it going away on its own are 50-50 if it is bug bite.
Generally when a horse gets kicked and receives a lump of thanks that big, the lump is soft and mushy, that's why I'm thinking some sort of critter bite.
Hope this helps:)