And as the years go by

There may or may not be a comic incoming. We'll just have to see.

I'm aware Spike doesn't grow wings in the show, but we've seen he can grow into a bunch of different things. For instance, his growth in the Cutie Mark Chronicles was completely different than in The Secret of my Excess. For all we know, Spike's growth in The Secret of my Excess may not have been his natural growth.