https://gist.github.com/4832b508eab672f7f035b3f5f70b34b0

When a guardian is spawned nothing appears and the player can take damage.
https://gyazo.com/c6a606533c81cfac3258e0f7b0f0d3f5

Also you can't see rabbits
https://gyazo.com/5e768f37872f5960bd350f11ac564abf

UPDATE: Endermite's as well.

Neither rabbits nor guardians did exist in 1.7.x

How does ViaVersion handle seeing older mobs?
I know Protocol Support display them as other mobs.

ViaVersion does not have this problem. All entities existing in older versions do also exist in newer ones. ProtocolSupport is replacing them with old entities. I think I will do this in a future update.