That looks like lateral digital extensor tendon protrusion from upright conformation. If I’m looking at the right thing, it’s not a lump or a swelling. It’s the extensor tendon pulled tight as a drum because the entire bony column, from the coffin bone on up, is at too steep an angle. I’d wager that in a side view picture we’d see that this horse is excessively upright through the pastern and fetlock and possibly the foot as well.
Especially in young horses (but possible at any age), long trailer rides can lead to contracture of the extensor apparatus and flexural deformity of the joints. Sometimes it presents as you show here, sometimes as a sudden “over at the knee” and sometimes both deformities simultaneously. It is something you need to address immediately if this is a young, growing horse. 24/7 turnout would be my first step, Buckeye Foal Aide Paste, my second. Even if this isn’t a “baby baby” the Foal Aide will help. You need a competent farrier and a knowledgable vet working together to try to correct this horse’s angles ASAP.
Is the horse knuckling forward at the fetlocks at all? If not, I can tell you from the pics that he’s not far from it. Was he tending to be upright before the trailer trip?