Chris Brown -- Completed Head Tattoo ... It's In the Stars! (PHOTO)

Chris Brown's decided one ridiculously large tattoo on his cabeza isn't enough -- unveiling the sequel to his Greek/Roman goddess ink, but this time his inspiration seems more obvious.

CB got a big ole bull inked right next to the Venus de Milo(ish?) chick he got the other day, and captioned his Instagram ... "I do what the f**k I want!"

We don't know if Chris is big on astrology, but he IS a Taurus ... so the bull makes as much sense as a head tatt possibly can.

At this point, even Mike Tyson is cringing.