FTH said:
But who knows, there's no telling how many types of ponies there are out there!
If it's based on the original series there are also sea ponies (sea horses basically) and flutter ponies - which are smaller and have wings like Rarity's in the Sonic Rainboom episode; translucent and delicate.
If you want to go into the toy line, you've got all kinds of crazy subsets like the 2x fancy ponies, so soft ponies, petites, dream beauties, windy/summer wing, twinkle eye, princess, sweetheart, mermaid, flower fantasy, sparkle, glow, pocket, magic message, sweet berry, perfume, candy cane, etc. The list goes on, don't exactly remember them all like I used to. Needless to say there are a LOT of variations.
They are mostly offshoots of the main types tho; Earth/Pegasus/Unicorn/Water (?), just with their own special look or talent. Like the magic message ponies had cutie marks that changed, or the 2x as fancy ponies had cutie marks which spread all over their bodies. Then you've got the weird ones where the cutie marks are actually extending from their bodies like the pocket ponies which had little items which went inside their cutie marks; or the princess line which had jewels/gold sticking out of their cutie marks.