I kinda noticed it too, but word of staff said that the only ever mention of "Derpy" and Derpy actually speaking is only in The Last Roundup.
NotShinta wrote:I imagined an awkward pause of dialogue as RD walked past Ditzy in the latest episode, however. It was probably nothing.
The only episodes that Derpy didn't appear in Season 2 are:
Episode 6 (Cutie Pox), Episode 12 (Family Appreciation Day), Episode 13 (Baby Cakes, however, there was a baby with Derpy's colors), Episode 20 (It's About Time) and Episode 22 (Dragon Quest)