Prior to the introduction of Mecanim, Unity used a simpler animation system. For backward compatibility, this system is still available. The main reason for using legacy animation is to continue working with an old project without the work of updating it for Mecanim. However, it is not recommended that you use the legacy system for new projects.
To import a legacy animation, you first need to 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.