This vintage G1  mommy or mummy charm is Secret Keeper. She's white with yellow hair and a light orange key for her symbol.

The  were originally available in  of the My Little Pony line in specially marked My Little Pony packages. There were 20 different charms to collect and they could attach to a charm bracelet available separately through a .

Secret Keeper is in good vintage condition. 

The pony is flat on the bottom and is marked "1984 Hasbro."

Measurements:

Height: 1 inch (2.5 cm)