The Bug
In the latest beta any added entity will have their geometry broken and some/ all of their textures will be either missing or they'll be using the wrong texture. The custom animations won't be played at all

Edit: As of 1.8.0.14 added entities have become invisible and their spawn eggs cannot be found in the creative inventory. Any previously existing spawn egg has turned dark and lost its name. /summon also doesn't work anymore

Edit 2: It seems the problem lies in the documentation itself. It hasn't been updated properly in some essential areas.

Edit 3: Example pack working in 1.8.0.14 (credits to @SolvedDev, @OSprintermax and @Nyanth_player): https://github.com/solvedDev/MysticCraft

Steps to reproduce

• Download and apply an add-on that uses added entities
• Load the world and summon the entity