Half of the new monsters introduced in Hellfire (the expansion to Diablo) are just things that were dummied out in the original Diablo.
There are several items dummied out of Diablo II— some apparently complete, but with the flag that allows enemies to drop them disabled.
Diablo III has the Mystic Artisan. She was supposed to be an NPC who could infuse items with targeted enchantments, but was removed late in development. Her model, voice work, and place in the story still exists as the NPC Karyna you rescue from the Spider Queen, with the only difference being she doesn't follow you after. The artisan exists as of Reaper of Souls, she is able to change an enchantment on an item for another.
According to Suda 51, Henry was going to be playable for long enough to fight three bosses in No More Heroes 2: Desperate Struggle, but ended up fighting one due to time constraints. The scrapped bosses, in the final product, are just said to be killed offscreen instead.