The Greek for 'bull' is tauros and with a bit of imagination, you can see a bull's horns and snout in the 'T' shape; hence the astrological sign ♉ for 'Taurus the bull'.
The Crux Commissa is a 'T' shaped cross; commissa meaning 'joined' or 'attached'. In Christendom, the name given to this cross is usually Tau, named after the similarly shaped Greek letter.
It is quite possible this form of cross was used to crucify Jesus. Matt. 27:37 says:
"And set up over his head his accusation written, THIS IS JESUS THE KING OF THE JEWS."1
The head of a victim crucified on a Crux Commissa would invariably drop forward below the shoulder line and any writing at the very top of the upright post would have been visible.
The more widely known names are the Tau Cross (St. Anthony's Cross) and the Forked Cross (Y-shaped Cross).