You can unlock doors, sneak past guards, climb the wall, but if you don't take out that monkey, you ain't going nowhere. You want to get out of here, get rid of that monkey!
~ Chatter Telephone's warning to Woody
The Monkey is a Musical Jolly Chimp Monkey Toy and the (former) quinary antagonist of Toy Story 3. He is a cymbal-banging monkey toy at Sunnyside Daycare. He doesn't speak, but he does make monkey noises. He has two cymbals he can bang together.
He is first seen being played with by Bonnie Anderson when the much older Ms. Davis goes to Sunnyside to donate the toys and meets Bonnie's mother, and Bonnie is extremely shy to talk to Andy's mother.
During the times when Sunnyside was under Lotso's control, the Monkey has worked under Lotso as a nighttime guard watching all the security cameras in his office. Every time a toy tried to escape, the Monkey would push a button that turns on the P.A. and screeches loudly into it while banging his cymbals together repeatedly, alerting Lotso and his other nighttime guard toys to capture that toy.
When Woody returns to Sunnyside to rescue his imprisoned friends, the Chatter Telephone tells information about the Monkey to Woody, and says that he'll have to get rid of the Monkey if he wants to escape.
That night, the Monkey is busy focusing his eyes on one camera where Ken, Big Baby, and the reset Buzz have cornered Mr. Potato Head (who escaped from his cell and left a real potato in it) he didn't notice Slinky coming out of his cell in the Caterpillar Room through another camera. Woody and Slinky quietly go through the vent into the Monkey's office. With Slinky grabbing hold of Woody and swinging him, Woody tries to put a plastic black bag over the Monkey's face, but the Monkey already noticed them, and quickly turns around and screeches at Woody, making him and Slinky fall on the Monkey due to getting startled. The Monkey and Woody both get up, and the Monkey turns and hisses at the surprised Woody and scatters off to go tell Lotso. However, before he can reach the door, Woody trips him over with his string. This then leads to a brief fight, in which the Monkey overpowers Woody by repeatedly banging his cymbals on Woody's face. Woody tells Slinky to grab a nearby scotch tape, and the Monkey screeches in horror when he turns to see Slinky charging toward him. Woody and Slinky then tie him up with the tape and lock him in a filing cabinet.
Lotso's later knowledge of Woody and his friends' escaping in the first place is not revealed, but it is likely that the Monkey freed himself and alerted Lotso, leading to Lotso beating the Chatter Telephone for information on Woody's whereabouts.
During the film credits, the Monkey has returned to his position of monitoring security cameras at night again after Sunnyside is revolutionized into a cool and groovy toy paradise by Barbie and Ken, Sunnyside's new leaders. Seeing a disco party taking place at the Butterfly Room through one of the security cameras, the Monkey, also enjoying the disco, puts on his star-shaped sunglasses, bangs his cymbals as if they were drums, and starts dancing to the music, signifying that he has reformed as well.
Though the monkey typically monitors Sunnyside to ensure that prisoners are returned to their cells if they attempt to escape, he did not notice Woody attempting to do so. This is presumably because he was being played with in the Butterfly Room at the time.