Here's a list of characters. Let me know if you want anything added. Each base contains a unicorn and pegasus and alicorn parts, except the stallion and creatures.
MARE BASES:
1. regular mare
2. tall mare (Twilight base)
3. alicorn mare (Cadance / Fire Flare / Fleur de Lis base)
4. tall alicorn mare (Luna base)
5. not quite a horse (Celestia / Twilight base)
6. regular filly
7. tall filly
STALLION BASES:
1. regular stallion
2. heavy stallion (Shining Armor base)
3. tall stallion (Mudbriar / Flim / Flam / Starswirl / Cheese Sandwich base)
4. Colt
BABY BASES:
1. Baby 1 (Pound and Pumpkin Cake base)
2. Baby 2 (Flurry Heart Base)
CREATURE BASES:
1. Changeling (both regular and reformed)
2. Donkey
3. Dragon (baby, male and female by Garble and Ember reference, baby is Spike)
4. Hippogriff
5. Kirin
6. Seapony / Hippocampus
7. Zebra