I think I've deciphered your request...
1. user wants to download
http://www.mydomain.com/movies/name_abc.avi
2. name.avi does not exist in that folder (abbie: name.avi? what about name_abc.avi?)
3. i want the user to download a movie called "name_abc.avi" anyway (abbie: wtf?)
4. in the /movies/ folder there is an .avi called "name_xyz.avi"
5. user should download the "name_xyz.avi" but the name of the download should be "name_abc.avi"
If you're using unix go into the folder and type this:
ln -s name_xyz.avi name_abc.avi
That makes a symbolic link (abc) to your real file (xyz). No .htaccess necessary.