It looks a lot like Really big dinnerbone ponies. But why are they so big? What do they do? and why is there only legs?
These are part of the holograms. They are horses that are high above the air that give those messages at town spawn. I am not sure if that is the answer, but that is my best guess.
It is for the holograms ... the age set for the horses if it's really high or low...make their names offset from their bodies - so they are riding on top of a head and the heads are placed in the air.... causing their names (the holograms) to be where they are near the ground.... [horse] [head] | | v [horse name]
Aikar here (fixing something for Max) It will change in 1.8, as the horse method doesn't work, but Armor Stands have been made specifically able to support this, and mojang will give us a different entity in the future to even better support holograms.