First answer that pops into my head, the beast of the deceased mages. Will Set outlive his master? If so, I see no reason not to assume the same for the past summoned beasts.
Would explain his knowledge, powers, reason set is afraid (he is just like set, but with 1000x more experience and powers), why he doesn't intrinsically hate humans, and why he was so intrigued by the fact they saw him as "people".