Colossal Ethereal Dragon

Hunt time is the only time to find Colossal Ethereal Dragon, a Secret fish. It sits in an abundance to the left of the Crimson Cavern or the Luminescent Cavern while a Colossal Ethereal Dragon hunt is running, and Truffle Worm is the bait. According to the wiki, it is the only fish with two hunt spots in different areas as of 1.63.0. Hooking it is hard: it moves erratically across the bar, and reeling it in inflicts -90% Progress Speed.

Its ability is the real fight. Each frame it has a 30% chance to emit a deadly pulse, and that chance drops by 3% per attack until it reaches 15%. The pulse raises reeling progress by 25% over 0.8 seconds but sets the bar size to the current progress, so no progress can be gained. After 1.2 seconds it cuts progress by 35% over 1.2 seconds, again fixing the size, and the bar only returns to normal if the rod's passive changes it. The cooldown is 5.5 seconds.

Its progress speed can only be altered by the Overclocked enchantment, which requires Cosmic Relics. The wiki describes a large salamander-like fish with huge crystalline fins and antenna-like eyes, nearly identical to the Exotic Colossal Blue Dragon. The quips are "IT DOESN'T FEEL LIKE IT BELONGS HERE..." and "IT'S LIKE A DREAM FLOATING BY...".