The Angry Eyebrows are Awful Alvin's creatures of doom. Their behavior is similar to bats.
When someone is mad about something and doesn't let go of it, they seek for that person and attach themselves to the user. The only way to get them off is to let go of anger.
Many of the victims that had these were Ma Mushroom, Carrot Mom, Bob, Vicki, Junior, and Larry-Boy.
They were ultimately defeated by Larry-Boy when he lured them to the KnitMaster 3000 and turned them into a giant nightcap. They then trapped Alvin when he was angry about when happened. Ma Mushroom was the last victim to let go her pair, and the sole survivor of the eyebrows flew out.
It's unknown exactly if the eyebrows were natural creatures or if Awful Alvin created them. Though according to the story treatment, they were created by Alvin.