Your area indicated in the image is mislabeled, that is the proximal DDF flexor tendon sheath. Unlike your carpal swelling, the swelling of this sheath is common in older horses and often referred to as “throughpin (thoroughpin)” as it usually seen on both the inside and outside of the leg. Pushing on one side makes the other side bigger.
In short this is a common swelling usually not associated with lameness but you can read more about this at Overview of Diseases of the Hock (Tarsus)
DrO