Earth stallion, Unicorn mare, and Pegasus mare.
So... They're basically feathery wolves with hooves that walk on two legs.
Males have scruffier feathers than females. Pegasai have longer "arm" feathers (for gliding, not flying), rather than full wings. Unicorn horns are decorative (though alicorns can make sparks). Long manes become feather ridges down the neck and back, and longer tails to bigger tail-fans. Cutie marks are just coloured feathers.

#velocipony