Minecraft wiki entry for Development_versions, associated text: Use underscores to separate words, rather than using single words in mixed case.
Optionally, a minecraft: namespace prefix can be used, just like block and item IDs. However, it is not optional for EntityTags and spawners.[3]
There are still old entity IDs in the language files (example: LavaSlime instead of magma_cube for the magma cube) and the new mobs in 1.11 have their ID with the old type (no underscores, capital letters for the beginning of the words. example: VindicationIllager instead of vindication_illager for the vindicator.)
The following names have changed: