The Clay Golem is the first of the Necromancers golems, and arguably the most useful. Only one golem can be summoned at a time, and many Necromancers choose to go with the Clay Golem because of it sturdiness and slowing effect on enemies, making it an excellent addition to Skeletons making up the bulk of a Necromancer's army.
While it is fairly simple for a Necromancer to animate dead tissue, it is another matter entirely to instill the spark of life into inanimate objects. The Clay Golem is the simplest form of this complex art, creating a servant directly from the earth to serve the Necromancer. The intense drain this places on the psyche of the caster only allows him to maintain a single Golem of any type at a time.
↑Diablo II Manual, page 76. Blizzard Entertainment, 2000.