'Herp' = People who are enthusiastic and keep lots of reptiles are referred to as 'Herps', which is derived from Herpetology. Herp Derp has a reptile based cutie mark, so it's a play on words.

Derp came from the inevitable 'herp derp' internet meme. I was also being a bit silly when I made her, and the name just stuck when she actually became a thing.