Gronky was essentially one of the resulting creations caused by Kraid's attempting to collect a glitched powerup, combined with the glitch archive area of Zebeth. Crocomire Hunter found him wandering around, and took him as a pet. However, Gronky (having been "tamed" all of 10 minutes prior) attacked Samus' glitched body. He was then left on Zebeth while everyone else was transfered to ZR388 so Kabutroid could clean up the planet. Gronky survived this glitch cleanup, and ran off. He was later recaptured by Crocomire Hunter, only to escape once again shortly thereafter.
As of late, Gronky has been seen gronking incessantly at Boxy and Unspawny, and appears to have somehow spawned or otherwise befriended a smaller glitch creature, who has also taken to gronking at the trapped duo. The two have otherwise appear to have befriended Crocomire Hunter.
Gronky sprite credit goes to Jesse D.