IIRC, when you overwrote the bootanimation, it took the name of the stock file that was the bootanimation.zip. so, your new one wrote over your stock one, no.
If so.. that means you either better have (1) backed up the original bootanimation.zip or (2) need to find the original on the web so you have it to overwrite the current one.
Either way, the bootanimation.zip file is located in /data/local IF you were rooted it would be as easy as dropping in the stock bootanimation.zip file and you would be done. So, in your case, you need to track down the stock bootanimation.zip and then find a way to push it to /data/local without being rooted - like the app you used to change it originally
Someone can correct me if I am wrong, heck, it might be as simple as just deleting bootanimation.zip from /data/local and you get the stock one back. but again, root is needed for that access.
EDIT: I use the Droid X bootanimation and I see in /data/local bootanimation.zip file is there and I see no bootanimation.zip backup file on my computer - where I backup al files I edit - so that tells me I either did not back it up (which would be odd and very unlikely) or it means that bootanimation.zip just overwrites the stock one and maybe just deleting it would bring the stock boot animation back?