1. Select animation in your Assets folder
2. Expand the file with the little arrow in the circle
3. Select the animation itself, the icon with the play button
4. Press CTRL-D (on Windows) or ⌘-D (on Mac)

This will create a duplicate of that file that is no longer read-only!

https://discussions.unity.com/t/removing-read-only-from-animation-clips/689851/4