You can take this horse to a good lameness Vet. A good Vet will start at the hoof and go up with a comprehensive exam. He (she) should not have any preconceived ideas of what is hurting. That can get you in a lot of trouble.
If the exam shows the Vet where the problem is, then that is what will be x-rayed or ultra-sounded.
If the problem cannot be pin-pointed, then nerve blocks, progressing up from the toe, should be used to identify the spot. Never let a Vet start doing random x-rays unless he knows exactly where the pain is coming from.
If it were me, I would first flex that questionable ankle very tightly (flex no other joint at the same time). I would pinch the swelled area while it is flexed to see if the horse shows any tenderness. After keeping the fetlock joint tightly flexed for 1 1/2 to 2 minutes, have someone immediately trot the horse off. If there is a problem there, it will almost always make the horse markedly more lame. The horse may trot out of it, but those first few steps should be VERY lame.
If you want to try one last thing before spending several hundred dollars on a years late exam, you could put a hard blister on the area. I would use Reducine, available OTC. It comes in a blue can and is pretty remarkable stuff. It takes several weeks to do it properly. You clip the area and brush the stuff in with a stiff old brush (like a toothbrush). When it gets too tender to do (usually about a week) you give it a week off. Then, do it again. Do this repeatedly. It will blister but will not scar.
You can also get a hard, 1 time blister, like a red mercury blister, but I have found Reducine to be much more effective. [For those interested, it will completely remove a splint -- even very old ones.]
I also have a leg Vet that will do a deep injectable internal blister on some things. He has saved many horses with his procedure that were given up on by other leg Vets. He has done 2 of mine and one of those was lame for more than a year and had been seen repeatedly by a local Vet. She is sound and being ridden hard to this day and was injected about 5 years ago. I know of several that went on to barrel race at a World level and roping horses that went to the NFR after he injected them that way.