Alright, this is a tricky t-shirt to describe... most people think this song was written by the metal band Judas Priest (Album: Hell Bent for Leather - Rob Halford, Glenn Tipton, K.K. Downing)... but The Green Manalishi with the Two Pronged Crown is actually a Fleetwood Mac original... written by British blues rock guitarist Peter Green in 1970.
The song has been covered by many bands such as The Melvins (Kind Buzzo, Dale Crover, Joe Preston) on their 1999 album The Maggot... this cover is very cool I might add. It's also been covered as far back as 1983 by the metal band Corrosion of Conformity COC (Pepper Keenan - awesome guitarist, also in the band DOWN with Phil Anselmo) on their skate punk rock album Eye for an Eye.
Anyway, the song was written by Green after experiencing a drug-induced dream... where a green dog came and barked at him (sounds a lot like a Roky Erickson nightmare). Supposedly this green dog is to mean Money... represented by the devil... although it is often thought that the Green Manalishi is a form of LSD or other psychedelic drug.
Either way, this was the last song Green wrote before he departed from Fleetwood Mac... the band HE started... an enormous shame since Peter was on top of the world when his drug use eventually drove him mad (much like Syd Barrett)... finally leading him to the point of living on the streets like a crazy hobo... you may have walked right passed him not even knowing who he was.
So you can see... The Green Manalishi has some seriously rich rock and roll history... this amazing 70's rock song was conceived in a dream... thought to be a message from the devil... which eventually drove a brilliant guitar player insane... I don't think there's much more you could say about the cult factor of this totally cool Fleetwood Mac t-shirt design.