Horse's lower hind leg swollen, moves when pressed. What should I do?

You have some great observation skills about your horse!! It sounds like you are describing a joint that is filled with excessive fluid. Joints have 'pouches' that bulge on the sides of the joints when they are filled with fluid - and when you press on one side, the fluid moves to the other side. Structures on the hind leg that have obvious pouches like that are the fetlock joint and, in the same region, the tendon sheath. Up higher on the leg - another tendon sheath as well as the hock joint can have obvious pouches. The swelling itself may not be cause for alarm, unless it is brand new and/or is also accompanied by lameness (limping). If there is lameness, you should have Ellie evaluated by your veterinarian to determine if the swelling is associated with the lameness and what treatment should be done. If there is not lameness associated with that leg, mention the swelling to your veterinarian the next time they are out for an appointment or you talk with them next. Good luck with Ellie and thank you for using Pet Coach.