Sprite animation consists in showing an existing sequence of images or frames.
First import a sequence of images to the asset folder. Either create some images from scratch or download some from the Asset Store. (This example uses this free asset.)
Drag every individual image of a single animation from the assets folder to the scene view. Unity will show a dialog for naming the new animation clip.
This is a useful shortcut for:
Preview the playback in the animation tab by clicking Play:
The same method can be used to create new animations for the same game object, then deleting the new game object and animation controller. Add the new animation clip to the animation controller of that object in the same manner as with 3D animation.