Toitank

Toitank (トイタンク) are small gun enemies in Mega Man Zero 2 that shoot at Zero while riding on tracks on Phoenix Magnion's stage (Power Room). It comes in three different colors (purple, blue, and green), each with a different speed. If Zero defeats all of them before they disappear, a U4 will appear.

U4
U4 is a robot that appears after all Toitanks have been defeated. It contains Dable, a rare Cyber-elf. It zips by on a track at a very rapid speed, and requires an extremely well-timed strike to hit before it disappears.