Are you talking mentor spirits that have appeared mechanically in 5e materials, mentor spirits that have been mentioned by name in-setting, or mentor spirits that exist in the world?
If it's the first, Rooks has it. If it's the second, I can't really help you. If it's the last? Pretty much anything that makes sense.
A mentor spirit is essentially a product of the collective human subconscious, a product of faith, that has gained some measure of self-awareness and power, which it in turn uses to influence the collective human subconscious in a manner consistent with the faith that spawned it.
In some ways, a mentor spirit can be like a deity, or it can be a tool of a deity, or it can be an aspect of a deity. Or all three, in different ways, at different times, to different people.
Every being and idea of every faith in every way is, in some way, a mentor spirit, be it Coyote, Thor, St. George, a river spirit, the sanctity of life as an abstract concept, or your late, great great uncle Howard.
Whether St. George is a mentor spirit in his own right that just happens to have similar benefits to Dragonslayer, or St. George is one of the ways Dragonslayer manifests and the saint himself does not exist, or if St. George is an independent entity that acts upon the world through the medium of the psychic construct that is known as 'Dragonslayer' is unknown, unknowable, and it's a truth that's less important than what the individual character believes to be the truth.
