Bubbles are enemies from The Legend of Zelda series. The are flying skulls wreathed in either red, blue, or green flames.
Two types of Bubbles appear as enemies in Smash Run, those being Red Bubbles and Blue Bubbles. Red Bubbles deal fire damage and are weak to water attacks while Blue Bubbles freeze players and are weak to fire attacks.
Green Bubbles are also depicted on its Trophy, but do not appear in Smash Run.
A skull wreathed in flames. If that wasn't hardcore enough for you, in Brawl they'd leave a curse on anyone who touched them. In Smash Run, the red ones attack with shocks and are weak against ice, and the blue ones attack with ice and are weak against fire. Just keep that in mind when you're taking them on, and you'll be fine.