Tentacle Skull

From Homestar Runner Wiki

The Tentacle Skull is a skull with tentacles which rides a Penny Farthing at various points in Experimental Film. It is always animated in Powered by The Cheat style, and is mostly seen in an obscure sports stadium, and in the background of the kitchen with the blueberry muffin, but once pogos split-screen with Strong Sad (who is dressed as the Grim Reaper). Strong Bad states in the Experimental Film Commentary that "if it weren't for that tentacle skull riding the old-timey bike, this video would have nothin'!"

Skulls are a frequent TMBG theme. In addition to being mentioned in several of their songs, they appear on the cover of their Back to Skull EP and on the back of their John Henry album. The tentacle skull might also be the Skullivan, as mentioned in the companion EP for The Spine, The Spine Surfs Alone.