Woodstock's grandfather is a Peanuts character, who was referred to several times but only seen in a flashback. No name has been given to him.
Woodstock's grandfather was first mentioned on January 4, 1994, when Snoopy and Woodstock decide to go to visit him. They find a bird cage, with a diary in it. Snoopy reads the diary, and learns that Woodstock's grandfather was stuck in that cage for a long time. Every day Woodstock's grandfather wrote in the diary that he wanted to leave the cage because he hated it.
The diary ends with the grandfather saying he has been put in the sun, and wants to get out of the cage. Snoopy tells Woodstock his grandfather is watching over them, but then, so as to not freak Woodstock out, Snoopy adds that he means from a telephone wire.