What I got from this song, and the comments; This is the murder of someone, and feeding them to a horse while the other feels betrayed before/after said murder.

“The stalking horse was hides the guy” Stalker?
“Which the pony was a phony was a lie” Betrayal
“Go away” Could’ve been fear before being murdered
“It was catastro, catastro-feeling good” Said murderer may have enjoyed the process of the killing.
“Is in the bag which you drag behind your car” Possibly the dead body of the victim.