Josh and Zack Greenfield were brothers experimenting with enhancing dogs with super abilities using meteor rocks. They served as the main antagonists in the WB TV series Smallville Season 4 episode "Krypto".
Josh Greenfield worked as a trainer at LuthorCorp where they conducted animal testing, specifically dogs, mixing steroids and meteor rocks to create super-powered dogs with the aid of a special serum. The effects however were only temporarily. Seeing an opportunity, Josh stole the research and used it to create their own serum to enhance two dogs with superhuman strength from the animal shelter in which he and his younger brother, Zack, worked at. Having access to the serum, Zack and Josh began to use the dogs, Einstein and Hercules to commit robberies. The brothers trained the dogs to attack and be smart enough to steal for them.
During a robbery at a small town grocer, Hercules and Einstein rob from an elderly man. While Einstein opened a safe and takes the money out, Hercules savagely chased the grocer into the backroom, and had the force to dent a steel door. Zack and Josh collect the rottweiler and the money but Einstein the retriever ran away into the night and was found by Lois Lane. Lois brought Einstein to the Kent Farm where he displayed his unique abilities and quickly formed a bond with Clark. Lois and Clark realized there was something different about the dog and found an identification chip under its skin and they decide get the chip scanned at the vet.
The Greenfield brothers were running the shelter and Lois noticed that the dog growled at them and was reluctant to go with the brothers. They scanned the tag and the owner comes up as LuthorCorp. Zack mused that it must be some kind of lab dog and offered to return it but Clark didn't want to leave the dog there so he asked Lois to take it back to the farm while he goes to talk to Lex. Before they left, the brothers made Clark leave his name and number. Zack reports that Einstein has been found and was at the Kent Farm. Hercules was locked in a cage and Josh antagonized it. The brothers talked about the background of the super dogs and planned to get their dog back.
The Greenfield brothers sent Hercules to the Kent Farm to try to get Einstein back. During the dog attack, Jonathan was injured until the Rottweiler hears its calling whistle. It runs away and the retriever chases it away but returns to the farm. At the hospital, Lois points out that the only people who knew the dog was at the farm were Lex and the brothers at the vet and both decide to go investigate the vet's office.
Zack and Josh soon got the dog back, but this got them into conflict with Clark Kent. After committing another robbery and attempting to murder Clark, who was saved by the escaped dog, Zack and Josh were apprehended by the police.