(Tock-)Tick is an unseen character of the Phantom Tollbooth. He is Tock's brother. He is the son of Tock and Tick's parents
When Milo was asking Tock why his name is Tock when he ticks, Tock said that it was a long story. Before he was born, his parents named their first puppy Tick, but when they wound him up, he went "tock-tock-tock". So they named the next puppy Tock, expecting him to tock too, but he ticked. They tried to change their sons' names, but couldn't.