Living horses come in a wide range of sizes. Their heights are usually measured in hands. The largest is the English Shire horse, which can measure as much as 20 hands. The well-known Clydesdale draughthorse, one of the largest of horses, is seen here to dwarf one of the smallest varieties, a miniature horse. Despite the obvious difference in size between these two, they are both clearly horses.
Horses vary in other ways too.
Also, three-toed horses are known today. An important part of the biblical creation model is that different kinds of creatures were created with lots of genetic information. Natural selection can sort out this pre-existing genetic information, by eliminating creatures not suited to a particular environment. Thus many different varieties can be produced in different environments. They also had other controlling or regulatory genes that switch other genes 'on' or 'off'.