From Wiki

"The Green Manalishi (With the Two-Prong Crown)" is a song written by Peter Green and recorded by Fleetwood Mac. It was released as a single in the UK in May 1970 and reached #10 on the British charts. The song was written during Green's final months with the band, at a time when he was struggling with LSD and had grown withdrawn from other members of the band. "Faced with the band's refusal to give away all monetary gains, Peter Green decided to leave Fleetwood Mac, but not before writing the haunting 'Green Manalishi,' which seems to document his struggle to stop his descent into madness."[1] While there are several rumours about the meaning of the title "Green Manalishi", one referencing a mysterious LSD drug called the "Green Manalishi" associated with the drug scene of the 1970s, Green has always maintained that the song is about money, as represented by the devil.

Green has explained that he wrote the song after experiencing a drug-induced dream, in which he was visited by a green dog which barked at him. He understood that the dog represented money. "It scared me because I knew the dog had been dead a long time. It was a stray and I was looking after it. But I was dead and had to fight to get back into my body, which I eventually did. When I woke up, the room was really black and I found myself writing the song." He also said that he wrote the lyrics the following day, in Richmond Park.