I understand that some third party launchers support renaming applications. I do not actually know, for I do not use one. The default application launcher does not support renaming applications.
Other than that, the only way I know of to rename an application is to do a little hacking to the actual application binary.
Warning: Technical information follows. Mere mortals should turn back now.
Using a resource editor such RsrcEdit or Bird, find and edit the application (or the ovly, if there is one). Check to see if it has a tAIN 1000 resource. If so, edit this resource and change it's contents. If not, add it and give it the contents desired.