2a mimic that follows you
The pet chases you by spinning through the air on a trail of purple fire and spits out fake coins as it goes. It also glows while flying, so before you have found a proper light pet this covers the job, which is a strange thing to say about something that normally tries to eat you.
two ways to get one
Shadow chests in the underworld are the direct route, and the crates that share their loot table are the lazy one, so you can fish for it from a safe platform instead of walking hell. If you are already down there opening chests for the weapons, you are farming it anyway.
dyes
It takes dye like any other pet, and since the mimic sprite has several real counterparts, the right color turns it into a convincing hardmode variant rather than just a tinted one.
the joke
The full chain runs dungeon chest, golden key, locked chest, shadow key, shadow chest, ornate key. At the end of all that, what you have earned is a pet chest.