My Pretty Pony was a model horse made by Hasbro, about 10 inches high and made of hard plastic with a brushable mane and tail. Later Ponies were softer plastic and smaller.
When you pull a switch under their chin, the original ponies wink, flicker their ears, and twitch their tails in a highly pornographic manner. The plastic is sometimes scented rather disgustingly to match the pony's name.
The original Ponies, in 1981, were named Cotton Candy, Blue Belle, Butterscotch, Minty, Snuzzle, and Blossom. These quickly exploded into an unmentionably huge variety.
The names of the Ponies seemed to get dirtier and dirtier, perhaps invented by some Hasbro product managers with a sick sense of humor, or one who wanted to milk the sublimated sexual obsession that preteen girls have with horses. Lickety-Split, Love Petal, Cherries Jubilee, Ruby Lips, Lovin' Kisses, Heart Throb, Love Melody, Cuddles, Sugerberry, Twisty Tail, Squeezer, Tossles, and Romper are some of the most porn-star-like names.
They were made up until 1991 and are hot collectors' items.
Stephen King wrote a short story, "My Pretty Pony". There was an animated movie and TV series featuring the ponies -- unfortunately not related to the King story.