Some hostile mobs are invisible. They can attack, but they do not make noise and cannot be hit. Disconnecting and reconnecting gets rid of them.
Sometimes, if a mob is invisible to one player, others can still see and attack the mob, but there have also been occasions where a mob is invisible to both me and another player.
It might affect passive/neutral mobs as well, but I wouldn't have noticed because they haven't attacked me, and being hit seems to be the only way to know that there is an invisible mob nearby.
I've only encountered it in Realms, but I play very little on servers/singleplayer so it might occur there too. 

Edit: I forgot I was using OptiFine, so it's totally possibe that it's related to that, but I'm not sure if my friends were too, and they were experiencing the same problem.