Laminitis
Inflammation and separation of the laminae inside the hoof. Severely lame, with a strong pulse at the fetlock, and an emergency.
The laminae are the layers of tissue that suspend the coffin bone inside the hoof capsule, and laminitis is those layers becoming inflamed and coming apart. The hallmark is severe lameness with bounding digital pulses; the causes named most often are carbohydrate overload, taking too much weight on one limb while another is injured, and endotoxaemia. What separates a bad episode from a permanent one is whether the coffin bone moves: for the first 72 hours it is still where it should be, and rotation or sinking is what makes the case chronic. This is the entry on this whole site where reading a first sign early actually changes the outcome, which is why every reference on it tells owners to learn those signs themselves.