Passive mobs seem to alter between a visible state and an invisible state. They do not keep one state permanently (obviously how would I tell they were invisible) but sometimes can be seen and sometimes cannot.Sometimes the visibility returns when they move but this may just be chance.
Environment
Windows 7, 64-bit Operating System, Java 7 Update 67, Render Distance 3 Chuncks, Brightness-Bright, apart from these all video settings are set at the lowest possible options.
Duplicate of MC-65040