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.
An ant cannot be obtained as an item even by using the /give command. An ant can be placed using commands such as /fill or /setblock, and naturally generates in the "ant" dimension. It cannot be destroyed by tools or explosions.
Natural generation
One ant block naturally generates in the "ant" dimension, where it begins moving immediately.
One ant block naturally generates in the "skygrid" dimension, however it does not move
Usage
An ant simulates Langton's ant. If placed on top of black or white concrete, it will move in a deterministic pattern, turning the concrete underneath it into its other variation. The ant block is the Turing machine mentioned in the update's changelog.
If two ant blocks collide, they will merge into one. It cannot be pushed by pistons, and is blast resistant, and can remove bedrock.