Elytra don't render on mobs.
To replicate this bug, simply put an elytra on a zombie, preferable at night.
Use the following command:

/summon Zombie ~ ~ ~ {Equipment:[{},{},{},{id:elytra,Count:1},{}]}

And push the zombie off a cliff. The elytra works, but doesn't render. I guess we don't need the mob to go upside down and all, but he sould at least look like a beetle.
Sorry for the poor quality of this report, but it's almost midnight right now.