You mean the elephant koi? They're these huuuuuuge fish that live next to Kiyoshi island - bigger than Appa, or at least bigger than Appa back when he was Appa-sized - and they're red and gold and have these long whiskers that make it really easy to grab on and ride them.

[he punctuates his words with gestures, arms reaching out in wide arcs to encapsulate their tremendous size and fingers clenching into fists to mimic grabbing onto their whiskers like a set of reins.]