WTF!! Britney Spears To BF -- Bark Like A Dog -- 'Cause You're My Bitch

Britney Spears is into 2 things -- her BF and B&D .. and she got to enjoy both at the same time during her Vegas New Year's Eve concert.

Brit dragged David Lucado on stage at Planet Hollywood ... forcing him down on hands and knees, strapping him into a harness and dog collar and then parading him around on a leash.

David seemed to enjoy it enough ... it's one of the fun perks of being with Britney.

It's gotta be hard on K-Fed ... all those fond memories.