CLONE FORMS!
I'm betting those horns weren't really digitally added. I was scanning inked drawings on paper those days, and I assume the horns were no different. It would've been cool if past me deliberately didn't ink those on paper and only added them via computer, possibly using obviously 3D rendered horns instead of just drawn ones.
...
Note to self: Do something like that in a future comic.
Commentary added December 22, 2014