Mage Meadowbrook

Mage Meadowbrook is an Equestrian Earth pony who was said to be an expert healer from Hayseed Swamp who cured an entire civilization of a Swamp Fever plague by gaining honey from flash bees, getting it safely by disguising herself as their queen, living ages ago until disappearing, as her home is taken care of by her descendant Cattail.

Role in the series
It's revealed in the series the reason she disappeared: She was killed by a Black Dog, which is a dangerous and aggressive wild dog that, like in mythical lore, is an omen of death by being symbiotic to germs, infecting her with an incurable and quick sickness and killing her. However, she is also revealed to have saved an entire town from a Black Dog before by curing one of it's minor-spread and curable diseases before her disappearance.