Thoroughpin is swelling of the tendon sheath in or just below the hock. It's often caused by conformational weakness, and normally shows up after intense work. The swelling is retained fluid rather than inflammation per se.
Thoroughpin shows up as a swelling on either side of the leg and pressing on one side causes the other side to swell more - hence "thorough." Most horses with thoroughpin are not lame and show no sign of being in pain. (If they are lame it often means there's an actual injury to the tendon). Thoroughpin is sometimes treated by draining fluid, but often recurs. As it doesn't cause lameness it's normally treated only with show horses in which appearance matters and often left alone in pleasure and sport horses.