Welcome back! We recently did away with "unique" pet names, so anyone can use any pet name they want! A legacy name is a pet's original name, or the name it had at the time we changed naming systems. You can change an older pet's display name to whatever you want, but its legacy name remains the same (handy if you want some bragging rights for having the "original" whatever). :) The cleansing pools removes the legacy name, handy if the legacy name wasn't that great and all you care about is the display name. I hope that clears it up a bit!

[edit] Whoops, guess I'm running a bit slow today.