The Ponies of Equestria

How many types of ponies are there in Equestia?

I think seven.

Earth Ponies

Unicorns

Pegasus

Crystal Ponies

Alicorns (between 4 to 7. See number of Alicorns)

Zebras

Changlings

In "The Elements of Harmony" encyclopedic book on pg. 53 it describes Changelings as follows:

"Changelings are grotesque shape-shifting ponies who can morph their bodies into anything. These frightening creatures have fangs as well as insect features and translucent wings designed to scare even the bravest pony into submission."