I am sure that I know exactly how large horses are. One horse fits in one terrarium. That's why they are called terrariums. Terrarium stands for "one horse fits in here." It was their preferred containment method in the early Greenhouse and Potted Plant Period.
So it does. That is why the scientific name for a horse is "Terra-horsebeast," not to be confused with the "Aqua-horsebeast" or the "Avian-horsebeast." So only a land horse will fit in a terrarium. You'd need an aquarium or an aviary for the others.