This feature was exclusively part of a joke version and is not currently planned for addition to regular Minecraft. It is documented here on the wiki due to being officially-released content by Mojang Studios, even if not present in a final version.
Bottles of entity are items that transform players into the respective mobs when drunk.
Splash bottle of entity are a variation of bottles of mob that can be thrown.
Bottle of void is a special bottle to transform back to the original player. Right clicking with it on a transformed player will remove the transformation and convert the bottle into a bottle of entity of that transformation.
A transformation into an entity will sometimes lead to special effects. For example, transforming into a chicken will allow the player to glide slowly to the ground. The chicken will however take fall damage upon landing, killing it and transforming the player back to themselves.
Transforming into an enderman will cause the player to be teleported randomly when a projectile it aimed at it or when they're being looked at by other players. Transforming into any mob capable of flight (for example, a bat) will allow the player to fly in survival mode as if they were in creative mode.
Added bottle of entity, splash bottle of entity, and bottle of void.