The photos appeared on GSMArena, and it looks pretty legitimate. The device looks very similar to its predecessor, the Galaxy S5 Active (pictured above).
A heart-rate monitor can be seen on the rear of the handset next to the camera sensor, but it looks like the fingerprint scanner has been replaced by a regular home button (similar to what happened with the Galaxy S5 Active). It also features physical "recent apps", "home" and "return" keys, as opposed to the standard S6's capacitive buttons.
The Galaxy S5 Active was an AT&T-exclusive handset, and divided critics with its rugged, ugly construction. If the above pictures are real, we might be in for more of the same.