Koko the Clown introduced rotoscoping to animation. Rotoscoping is the process of taking live action film and animating over it to recreate more human movement. Koko was taken from a film of Dave Fleischer in a clown costume.
The Out of the Inkwell series is interesting in that it lightly explores existentialism in animation. The figure that the animator has created is sentient and capable of thought and emotions, but is still relatively at the will of the animator. He can interact with the real world in addition to his own world. And what I find fascinating about this short in particular is that Koko creates a new being and then does away with him.