Muscle is a one-time villain from Teen Titans Go!, only appearing in the episode "Birds". He is a mutated mockingbird, who, along with Flex often antagonize the Teen Titans.
He was voiced by Scott Menville.
Robin had to clean the chimney to the Titans Tower because something was clogging it. These things clogging it happened to be two mockingbirds, (who would later be known as Flex and Muscle), sitting in their nest, which they had built there. Robin tried to get them out of there, by yelling at them but the two pesky mockingbirds mocked Robin. Robin threw an exploding can of birdy-cide in there, to get them out and they evacuated just as planned. After the mockingbirds left, the chemicals of the birdy-cide started to make them mutate.
Before long, Muscle had become a big, orange, muscular bird who could speak English and live among the humans, while Flex underwent the same transition, only he was blue. The two birds became friends with the Titans, as they showed off their big muscles and became their friend. However, when they spotted Robin, the did not take too kindly to him as he was the one who kicked them out of their chimney. Although Robin strongly opposed to seeing those bird anymore, the other Titans wanted to keep them around.
Muscle and Flex partied with the Teen Titans in the Titans Tower, but Robin kept spoiling their fun, so the mockingbirds and the Titans unanimously agreed to kick Robin out.
It wasn't long at all before Flex and Muscle started to get on the last nerves of the other Titans by throwing huge, destructive bird parties and making huge messes that they refused to clean up. Muscle and Flex beat up Cyborg for eating an egg sandwich.
The Titans were alerted to another chimney clogging and the mockingbirds forced the Titans to sweep the chimneys. The Titans saw Robin, sitting in the chimney in a nest, as this is where he moved to after being kicked out. The Teen Titans allowed Robin back onto the team and they all agreed to take down the mockingbirds.
The Teen Titans broke into the Titans Tower again and got into a huge fight with the mockingbirds. Unfortunately, the mockingbirds were too strong for the Titans to defeat in battle, so Robin threw an exploding can of birdy-cide at them, thinking it would kill them. However, it only mutated them even more, turning them into super genius future birds, who were evolved way past the forms of their former selves.
Flex and Muscle said that they had now been enlightened with the massive amount of intelligence they gained and that they had now gained inner peace. They had decided to stop being evil and peacefully surrender to the Titans in battle. Plus, their gain in mental mass caused them to lose a lot on muscle mass. Instead of forgiving them, however, Robin attacked them brutally, knowing that they were too weak to fight back. The two mockingbirds freaked out and cowered in fear as they fled the Titans Tower for good. Robin curses them as they left and claimed victory against the mockingbirds.