Chicken set was removed from RuneScape after an update, but still exists in-game for those who had obtained it.
The Chicken set was a Holiday reward received from the 2007 Easter event. It was considered one of the most generous rewards from an in-game event, and became an enormous hit when the set was released.
If the "Flap" emote has been unlocked from the Stronghold of Security, when wearing all the pieces of the set, players can do an enhanced "Flap" emote, in which their avatar lifts off the ground slightly as if flying.
If the enhanced flap emote is used, the player will have the pre-HD animation pose after the emote finishes, then quickly turn back into their normal HD pose. When this happens, it is revealed that the player models got one inch taller than they were before 1 July 2008.
When talking with a citadel guard in full chicken suit, the guard will exclaim, "Aha! Another one! C'mere, chicky, I've a wizard paying a bounty per head on you!", alluding to Avalani and her quest for chickens as test subjects on the clan citadelportal.
Similarly, when talking to a Citadel guard wearing any piece of a chicken set, the guard will say, "Are you trying to fool me? That's ridiculous!"
If you talk to Ava in Draynor Mansion wearing the full chicken set, she thinks you are crazy and will not talk to you until you take it off.