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In Unity (current version) we have two possibilities to make 2D animations: sprite-sheet si bone-based.
In the first image, the character is presented in several positions, with a sequence of positions for different actions. This image represents a sprite-sheet, being very popular in 2D games. This allows you to clearly imagine how the game will move.
In the second image, the dragon is divided into several parts of the body (head, body, arms and so on), requiring a newer 2D animation technique, commonly called bone-based animation. As the name suggests, the animation will be bone-based, in which each bone in the body can have a specific action or animation. With all the main body parts separate, it allows developers to create animations directly in the editor. This new animation technique is very similar to the one used in 3D animation.
Making a 2D scroller game