The animations take the pivot of an object its center. As could do to make the animation takes as a pivot point of the object, a point that I want to do the rotations?
If you are using an external editor such as Blender to define your objects, you can usually change the pivot point of the native object in such an editor. If you have no ability to change the model, then you can achieve your effect by making the object you desire to be rotated a child of an Empty Game Object. Position your model object such that the empty parent exists at the pivot point location desired. Once done, if you now apply an animation to the empty game object, it will rotate the empty about its point location which will rotate the child (game object) also about that pivot point.