Adds One-One from infinity train!

To obtain him:

Throw an ender pearl, smooth quartz block, and redstone dust on the ground together. This will give you a One-One Orb. Use this item near a tamed, unnamed dog to transform it into One-One!

Usage:

Right click on the top of One-One's face to split him into Glad-One and Sad-One.
Right click on the top of either Glad-One or Sad-One's face when they are near each other to recombine them.

Warning: Having multiple One-Ones is supported, but allowing them to be very close together causes some glitchy behavior.