Is NOT chosen for speed.
Most often used in rocket charges or simply slamming, bashing, or bringing down 2400+lbs of pressure on his intended targets.
It also drains the most calories, and is often seen eating something nearly all of the time while in this form for prolonged periods.

Transforming into this form frequently, made his normal form retain:
Wide hips
Huge butt
Horse like strength, and it shows in his legs and neck.
Smaller feet
Bigger nostrils.
Better Digestive system, which consequently partially divided, enlarged his stomach, his alligator form made his guts stretchier
Better stamina
Bad gas
and
a better sense of smell and hearing

[backstory]
He love horse races, and attended events involving rangalodons horse racing, as horses. In one competition, May, Rhona, and his father competed against him. Later sabotaging him by spiking his food with super fattening grains and drugs. He could not win the race because of the drug taking effect making him slow down too much, but later took revenge as he used the extra fat to enlarge both his overall size and muscle, out running May, and trappling over his pop. Rhona was the only one who got away the lightest, or maybe taking care of a and sleeping next to a large horse with bad gas that was still under the super fat drug effects for the next week, which made his bad gas and already large size 5 times worse, was the worst one. (Trip could not return to his normal form cause of the super fat drug would make him 'DA BLOB" at his size, so he stayed in horse form to counter the calorie gain)

Later one he since been using his super sized form for a verity of heavy duty work and crushing and/or gore them in the process.

His bad gas is worst in this form....

Why not a rhino?
Because he likes horses more. That is the only reason.