At some point before the events of the series, the author of 2 and 3 came across and studied them, recording their powers, appearance and weaknesses in said books.
In "Tourist Trapped", Dipper mistakes Norman for a member of the undead; by the end of the episode, it turns out that he is actually a small group of gnomes stacked on top of each other.
In "Little Dipper", Gideon Gleeful briefly considers summoning zombies to exact his revenge on the Pines family, but decides against it because they "don't take orders".
Later in "The Deep End", when Mabel is telling Mermando that she has never met anyone like him, she mentions the undead, referring to Norman in "Tourist Trapped".
In "Scary-oke", Dipper, in an effort to prove that the information in Journal 3 is real to a pair of government agents, foolishly summons a hoard of Zombies that attack the Mystery Shack. Later in the episode, it is discovered that the zombies weakness is three part harmony so Dipper, Mabel and Stan Pines sing to defeat them.