It is a purple horse made of jelly with four very short legs that barely touch the ground - as its belly is what touches the ground in larger part. As a result the legs look more like small flippers than actual legs. The Jelly Horse also has a short, stumpy tail and a few jelly protrusions coming from its head. The bits on its head are likely its ears and mane, though they are very simple-shaped. The feet have no visible hooves. All of these differences make it very different from a normal horse anatomically.
The Jelly Horse wears light blue reins that have a red berry with two leaves where they connect under the horse's jaw. The chariot it pulls is a tub full of milk, used like the milk suit to help Princess Bubblegum recover from the effects of the potion in the Boomerang bottle. The chariot is decorated with fruits and leaves like the horses reins, and is pink like many of Princess Bubblegum's things. It has pink donuts for wheels.