Midosu

The Midosu (ミドス) are a group of jellyfish-like boss Reaverbots from Mega Man Legends 2 found in the Nino Ruins, in the Kimotoma Caverns, and on Elysium. They shoot yellow homing energy balls and if there is water in the room, they gain additional abilities. They can also stun Mega Man if he collides with them without the Light Barrier.

Nino Ruins
Strangely enough, they are not fought as the main boss for the key to the Mother Lode (Klaymoor instead fulfills that role). This trio is encountered at floor B3, near the room with four giant Norieibis. If Mega Man Volnutt drains the water in this floor, the Midosus will only attack with the homing energy balls, and will not be able to move from their spots. If there is water, they gain additional attacks (see Kimotoma Caverns below). The strategy is to just circle them as Mega Man shoots with his buster, or if the player is feeling confident, getting closer to them and using the Drill Arm on them.

Kimotoma Caverns
Since there is no control panel which can be used to drain the water, Mega Man will have to fight them while submerged under water. In addition to the yellow homing energy balls, they can ram Mega Man, turn invincible for a short time, and if there is only one of them remaining, release miniatures of them which explode when touched or when enough time has passed. While circling them is still an option, be cautious of the other Midosus that may try to charge at Mega Man. The room they're guarding contains a red Refractor S.

Elysium
While they fight like they did on Nino Island without the water, it should be noted that gravity has been increased in the room they appear. This can make running away from their energy shots difficult.

Trivia

 * The Shield Arm weapon supposedly absorbs the yellow shots fire these reaverbots.