Our big horse had a bad reaction to "something" recently and suffered a bad dose of Lymphangitis - very swollen legs. It is incredibly painful and can cause them to run a very high temperature/heartrate. Keep an eye on your pony as it came on incredibly fast, and the sooner that it is dealt with the better. Is he lame with it?
Ours required immediate attention from the vet, anti inflammatories, painkillers and drugs to make him pee a lot to get rid of the fluid.
Have you tried cold hosing his legs to try and get the swelling to go down. Our boys legs swelled up by 5cms each at the front and 7cms each at the back!!!!!!
Within 3 days he was fine again, thank goodness.
Pop a tape round his legs and keep a record if you are worried, then you can see if they are going up or down!