UFO

The UFO only appears during the “Aliens” admin event, and the wiki says it can be fished up anywhere while that event runs. Its bait slot is just an alien emoji, and the catch line is the same emoji.

Fighting it is hard. The fish moves erratically, the reel suffers -99% progress speed, and neither the Steady nor the Blessed Song enchantments can help. In return it gives 11340 base XP, 45th of 101 Special fish and tied with Golden Egg, Gygax Egg and Puzzle Egg (Supersized Mosslurker gives 11320). The price is 2.836 C$ per kg, and that puts it 39th of 59 Special fish, behind Corrupted Kraken (4) and Incredibly Large Disco Ball (3) and ahead of Palm Tree (2) and Admin Crate (1.25).

The wiki describes the classic alien spaceship, with a giant green ray underneath that pulls its targets inside. It also says the model is based on the official Roblox catalog UFO hat, in which the saucer appears to vaporize the wearer's head.