I say go for it.
As ScytheKnight said, Mentor Spirits aren't really defined, they can really be anything. If the spirits are actual mythical creatures they might very well have been around back then. If they're not, but a construct of the Mage's subconscious mind, then it could simply create a vision like that. If it's something else, like a force of nature that was created with the Awakening, it could still make up that vision as a warning "that if somebody doesn't do something it will get worse" or so.