From Dungeons and Dragons: Monster Manual I, a popular undead encounter was the Skeleton. Animated by evil magic-users and clerics it was a common challenge to low-level heroes.
“[Skeletons] are found only in burial places or dungeons and similar forsaken places.”
Made more popular from the film Jason and the Argonauts (1963), Skeletons were a fun encounter. Mindless, yet hateful, Skeletons attacked the party with the same ferocity as seen in the movie. “Kill, kill kill!” is what they do. According to the Manual, Skeletons were active puppets to an evil animator nearby. They guarded crypts, cemeteries, and dungeons. However, I never followed the book to the letter. Often they were without a puppeteer nearby. They came to life when the living were too near. The creature was a good foreboding symbol that the party was approaching an evil place.
They were a good challenge to low-level heroes. Many would start off with characters of great fighting skill armed with edged weapons. The Skeletons’ natural defenses against blade weaponry frustrated adventurers who had to rely on high damage rolls.
Trivia: Regardless what weapon a Skeleton uses, it causes 1-6 hit points of damage. Holy water inflicts 2-8 hit points per vial. They were worth 14 XP.