First of all, it's "Steef." "Steff" is the name of a really bitchy girl at my school.
Second, let's get the facts straight. Evolution would take thousands of years. The Steef population was probably lowered faster than that, and all of the Steef on Sekto's wall have horns.
Evolution can't just happen. This isn't like Poke'mon, something doesn't just evolve whenever it wants or when its EXP grows to a certain point.
Stranger isn't "an evolved Steef." He's more likely just an adolescent.
Sorry to come down on you, but I hate it when people think that evolution is some instantaneous process. Your first theory was most likely correct.
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