Explanations:

A - Fits for earth pony due to his slick legs
B - B is for breezie
C - Slick movement similar to a hippogriff
D - D is for donkey. He's goofy
E - Idk
F - His teeth are sharp like a bat pony
G - His mouth is like a beak 💀
H - He's obvious, bro
I - His dot tittle triggers his horn
J - Same as I
K - Changelings don't have eyelashes, except the queen. She fits to be a changeling.
L - Levitation powers
M - Same as A
N - Same as A
O - He's speedy like a pegasus
P - Same as O
Q - There are no "snail-pony" hybrid in MLP universe. So I chose an abada (Central African mythological creature) for Q. https://en.wikipedia.org/wiki/Abada_(unicorn)
R - Same as A
S - https://en.wikipedia.org/wiki/Inkanyamba
T - He's obvious, bro
U - Because of his descendant, Waw (hook) https://en.wikipedia.org/wiki/Waw_(letter)
V - Same as U
W - Same as U
X - Kirin is an East Asian mythological creature. X likes East Asian culture
Y - Same as U
Z - He is so gigantic and powerful. Z is for zebra. Z is also fits to be an alicorn.