PETTROUT...Good guess on the key chain; however, (Beavis and Butt-head is an American animated television series created by Mike Judge with Texas tie-ins to Judge's later show, King of the Hill. The series originated from Frog Baseball, a 1992 short film by Judge. After seeing the short, MTV signed Judge to develop the concept.[3][4] Beavis and Butt-head originally aired from March 8, 1993 to November 28, 1997) - Wikipedia

With this information it highly unlikely to be what you suspected although we do know that the boys do have a way of time travel. I believe the little insult box was a small black plastic device and plainly marked...this show took place when I was in 6th grade + I def. owned one of those little insult boxes for a brief period in 5th grade until it was confiscated by my teacher...no doubt to probably play with it @ the faculty meetings, but that's neither here nor there...I've been trying to find out the name of the damn toy for quite some time w/ no luck...I've tried the internet...maybe I need to dig deeper...for some reason "The Eliminator" comes to mind as a possible name, but I don't think I'm quite right! Can anyone help a nostalgic brother out?