Thunder Slimer

Thunder Slimer is the mini-boss of Spark Mandrill's stage that is incased inside a huge blob.

Strategy
In both games where he appeared, Thunder Slimer will shoot blobs, making Mega Man X unable to move when he's standing there. Thunder Slimer will move on to a space and will shoot thunder bolts at X. It'll also create a bigger blob to let it go up. Thunder Slimer will then repeat again.

Another strategy is to beat Storm Eagle first and then use a fully Storm Tornado at it, killing it quickly.

Other media
Thunder Slimer appeared in the manga, Rockman X.

Trivia

 * In Mega Man X Collection, Thunder Slimer is green instead of blue.