User blog comment:SpongeBobLove/Half and Half Horse/@comment-10278667-20120804161842

I imagine it's the horse's blood, as horses do bleed when wounded. The scale of the wound might have caused blood that size. Also, when wounded, the wound only lasts for a short time before it seems to evaporate.

It was likely the horse was wounded. However, if that's the case, nothing short of some sort of glitch will allow you to keep that permanently, as wounds disappear after a short time.