Grunt met Nudge while being held captive in a circus and they became close friends and escaped together.  They formed a special connection and Grunt would always protect Nudge when she was in danger.  Later it was discovered that Grunt's brain was from one of Nudge's schoolmates, Henry Bucher.  Henry was kidnapped by Dr. Rhonda Lorre and then had his brain transplanted against his will into the body of a genetically engineered four-armed ape that the doctor had created for just that purpose.