When a 17-year-old St. Petersburg, Fla., girl did a good deed by taking a shelter dog for a walk, she didn't think the pup would return the favor.

But Mabeline, a Rhodesian Ridgeback mix, did – and then some. On a recent Saturday afternoon, the volunteer took the then-homeless dog for a standard walk near Friends of Strays animal shelter.

A registered sexual predator attacked the girl, grabbing her by the hair and pinning her down on the ground.

That's when Mabeline, who had never before displayed any aggression or guard dog tendencies, started barking until the attacker backed off.

WATCH the video below or READ the story at WTSP.com (via People)