I image you love your horse dearly, you life evolves around it. I would suggest this dream represents a fear of losing what you have come to love and depend up on for pleasure. A little insecurity, in your subconscious mind. Because a leg was still present this is a limb and represents attachment to that it belonged to. The fact that it wasn't attached means you are wondering, fearing, thinking about the relationship of you and in this case the horse. You see a dislocated leg means relationship dis attachment in some respect. Overall I would say the dream represents the subconscience: a fear of losing something, apprehensive about a change and questioning your relationships.