Symbol
From My Little Wiki
The feature that separates My Little Ponies from all other toy equines is their symbols.
Generally a rather simple design on repeated on both hips of a pony and confined there, but many variations have been made. From all over both sides of the body symbols, Twice As Fancy Ponies, or sculpted saddle ponies like Merry Go Round Ponies. By symbol is the number one way collectors identify My Little Ponies. But since there are so very many ponies to do saying 'it's got a tree on it', is too vaugue. So a visual assessment is necessary.
In G3 Hasbro more officially recognized how important this pony attribute is and began referring to it as a cutie mark.