Super Beast Imitating Drawing

By infusing his chakra into the ink beforehand, Sai can draw objects with a brush on his scroll (something he can do very quickly) to animate the ink directly off of the page. These creations grow to life-size once they have been brought to life--similar to Deidara's Exploding Clay constructs. For discreet communiques when he is undercover, Sai is able to turn the words he writes into small animals, allowing them to travel to their intended target unnoticed.

Since they are made of ink, a single blow is usually enough to dispel them. The one exception to this are snakes, which are used as living ropes to apprehend a target; they have been shown to be abnormally difficult to tear.

Here is a list of things that he has been shown able to draw:
 * Mice; usually used for tracking or searching.
 * Lions; usually used for direct attack.
 * Birds; used for flying on their backs, to search above ground. Used for long-ranged transport as well.
 * Leeches; used as a projectile-like attack, not doing much damage however.
 * Giant Leeches; able to be painted and summoned one at a time, this is just a larger and heavier leech.
 * Bats; being able to use them as umbrellas.
 * Fish; only seen used to deliver a message in water.
 * Snakes; usually used to constrict and bind opponents.

Influences
This technique is in reference to the Chōjū-jinbutsu-giga (lit. Animal-person Charicatures), or Chōjū-giga for short, a set of four picture scrolls belonging to the Kōzan-ji Temple in Kyoto, Japan and sometimes credited as the first manga. The scrolls depict various anthropomorphic animals dancing and playing about in a lively ceremony, which seems to be the basis for Sai being able to animate his own drawings for various purposes.