Hard lump on inside left fetlock hind leg

Hello Louise,
Is the lump in or closely associated with the skin or is it under the skin? To answer this question see if it moves with the skin or does the skin move over it. Under the skin in the area you indicate in your image is the junction of the medial branch of the sesmoidean ligament where it attaches to the top of the medial sesmoidean bone all of which is covered with the annular ligament (see the topics in https://horseadvice.com/horse-equine/reference-material/equine-illustrations/muscoskeletal-anatomy-and-conformation/lower-limb-anatomy-front-and-rear-limbs/).

So the first step is deciding if the lump is in the skin, just under the skin, or associated with the ligament/bone. The lack of lameness suggests this is not traumatic or inflammatory leaving degenerative or neoplastic disease processes most likely. Once localized it is probably going to take imaging or biopsy for diagnosis. If I was given 3 to 1 odds, I would bet on degenerative suspensory ligament disease…what breed and age is this horse?
DrO

The next step is to characterize what tissue(s) are involved and this will require radiographs if it is bone (disease of the sesamoid), ultrasound if it is soft tissue (disease of the suspensory branch or annular ligament), or perhaps a new growth (neoplasia) associated with these tissues or their surroundings. By lifting the leg and cradling it in your lap (like your farrier would do) so the suspensory is unloaded you may be able to better palpate the deeper margins associated with the swelling.
DrO