Other responses mention the lore, but historically the lore was written to explain an immunity that was already in the game.  Historically, elven immunity is a nonsense addition that came at a time when a lot of people were piling nonsense extra powers onto elves so they could be the Elder Race out of Tolkien.  The game became unbalanced and has needed several revisions to moderate much of that nonsense.  I wish 5E would remove this trivial ability.

These are the classic zombies which eat flesh, like the ones from Day of the Dead. 

Wait, they can talk? I've been running them for years and had no idea they can talk! lol

cause elves are like dat

Genetics lol .. it's an old school benefit that goes way back into the early editions.

Zombies could be a good idea. 2 years late to this, but I am just now using Ghouls in my Campaign and I have bunched 2 Ghouls (CR 2 each levelling) with some minor zombies (CR 1/4 each). If you are still looking for a suggestion, here is the best I can do.

thats nice     

No, because they are dead.