What is one of the four original types or prehistoric horses and ponies that was thought to be extincted?

The prehistoric horse that was thought to be extinct but was later rediscovered is the Equus przewalskii, also known as the Przewalski's horse.

This wild horse species was once found across the steppes of Central Asia, but it was thought to be extinct in the wild by the 1960s. However, thanks to captive breeding programs, it was reintroduced to the wild in the 1990s. Today, the species is still considered endangered, but it is no longer considered extinct.