A 1987 CD I own, manufactured by Castle Communications, and titled, 'Fleetwood Mac - The Collection,' lists the title as "SomeONE'S Gonna Get Their Head Kicked In Tonight."
Rikky Rooksby's 1998 book, 'The Complete Guide To The Music Of Fleetwood Mac,' also lists it as such.
However, both Mick Fleetwood's 1992 book, 'My Twenty-Five Years In Fleetwood Mac,' and Peter Lewry's 1998 book, 'Fleetwood Mac - The Complete Recording Sessions, 1967-1997,' list the track as "SomeBODY'S Gonna Get Their Head Kicked In Tonight."
I guess, out of habit, I still refer to it as "Someone's," because that 1987 compilation was my first exposure to the song.
Johnny Stew