It's a huge purple jellyfish with a Chick on top of it, which has implanted a mind control machine on it.
In the Christmas edition, the area where the Chick used to be is covered up with a Christmas hat, and two Chicks (as gingerbread cookies) are behind it. In the Halloween edition, it doesn't change at all.
It's encountered only once at the end of the Epsilon Thalassus: Artifact Recovery Mission chapter. There, it has 120000 health. After dying, it gives 100000 points, 12-15 Coins, and an Atomic Powerup. Its pattern isn't very complicated. First, it arrives from the bottom of the screen and uses Straight Neutrons. After that, it swims up, and reappears from the right side of the screen, performing the Head Dive attack. After it swims to the left, it reappears from the right side of the screen, and performs the Spread Neutrons attack. Its pattern ends with it swimming up, and reappearing from the bottom of the screen. It will then perform the Straight Neutrons attack again. A Satellite is summoned at the beginning and at the end of the fight. It is weak to the Lightning Fryer and it has no strenghts.
Focus fire on the Chick that's controlling it, as that is its weak point. It's better to move out of the Jellyfishes it spawns' way, as they aren't worth killing. If you have problems with avoiding the Spread Neutrons attack, you can retreat behind the Jellyfish King. You will be safe from it here.