The one to the right is vestigial. It has no other teeth besides the tusks. The largest incisors are the tusks of elephants and their extinct relatives, the mammoth and mastodon which died out at the end of the last Ice Age. The tusk incisors grow from persistent pulps and continue to grow throughout life. The adult tusks have deciduous predecessors.
Horses have three incisors per quadrant.
Their unusual design and wear throught life permits their use in judging the age of an animal. When the teeth first erupt they have a central pit.