Unity's Legacy animation system (Legacy) has a limited feature set that predates Mecanim. Legacy is still available because it is easier to use and provides better performance for simpler animations.

Legacy is also available for backwards compatibility with old Unity projects. Legacy uses the Animation component and the special Legacy import option.

To import a Legacy Animation, select Legacy as the Animation Type in the Rig tab of the Import Settings window:

Make sure you enable the Import Animation option on the Animation tab.