It doesn't matter if it's written. It matters on how it's supposed to be read. Bob said "An herbivore" He didn't say, "A herbivore" therefore it was written how it would have -sounded-. And honestly, not everything needs to be written to the most extreme grammatical correctness. Especially when it comes to a comic strip.
I like how people argue about pronaunciation whant the TEXT is WRITTEN!
Seriously... In written language you only use "an" only before a word that starts with a vowel. (period!)