You can't say for certain that there wasn't an artist who quick sketched a male version of Evelynn, especially if they were tossing around the idea of her her feeding off of both sexes. It could easily have been something silly someone did that could also have lent actual ideas to her finished project. In brainstorming and sketching, anything is possible, especially with a desire demon that feeds on humans. It's not something that is that absurd as to not have been toyed with at some point in her design, even if it was just for fun.
And again, non-canon art from Riot is actually more interesting, because that means even if it was a simple, silly sketch, it could have been a part of the design process somewhere, unlike a random artist. If, say, a concept artist made a silly picture of a male Evelynn as a sexy, tall demon butler who oozed sex appeal, with prehensile coattails, that could have been an inspiration for her lashers. That's just a bullshit example, but it shows you just how silly things can inspire the actual brainstorm process, and I know in many artistic communities, they encourage their designers to get crazy and silly before they get serious, as the craziness and silliness can more often than not inspire the other artists to go down paths they wouldn't have. So don't knock sketches and non-canon art from the designers. It will always be interesting and most often always have had some place in the design process.