Monkey is the dog that Dan gave to Chuck as another way to relieve his psychological pain from losing Blair. During Chuck's "path to being good" in Season 5, Monkey is seen most often in Chuck's Hotel Empire penthouse and walking with Chuck in Central Park.
Chuck first refers to Monkey as a mutt but grows to love him. Blair got angry at Chuck when she saw a Gossip Girl blast of him giving the dog away but later it's revealed that Chuck was only getting Monkey neutered. Monkey and Chuck seem to have a very close relationship with eachother. Monkey is friendly and cute towards Dan and Nate, but he can be very vicious at Chuck's command. Monkey is described as very cute in every way possible.