Entity 303 is a evil character who is possibly real he acts the same in videos as he would in real life he only appeared in if you could stack unlimited items also in real life would be a minecraft computer virus.

How Herobrine said in the vids of 09Sharkboy (Herobrine hunters series ) Its an acount of one minecraft creator which Herobrine hacked.

Never ever play minecraft at the July first (or u will get hacked like 09Sharkboy by enity 303)