Someone else may be better able to help, as there may be multiple ways to do this. However, here's what I can find.
The Wyvern is the Dragon, art file is dgnuwXX.art, located under unique npcs, in uniquenpcs.mes number 20...if you wanted to change that.
The palette as far as I know is actually contained in the art file itself. The palettes are located in hex starting at addresses 84, 484, 884, and C84.
They occupy 1024 characters.
I'm not sure exactly what your goals are, but if you say, had a different set of 4 palettes to load in, that worked for those bitmaps, you could copy them out of the bitmaps(if you start the palette with 0000ff(seen ff0000 in the hex editor) you can find it, it's pretty early in the file) and into the art files using xvi32. Also, in conjunction with artview you can load in a new picture, if that's what you wanted to do, by changing the palette manually first, then loading the picture in artview.
This is the method I've used so far to create new art(which I've done minimally) there's probably a better way out there than my hack.
Anyways, if this isn't what you want to do, or you do want to do it and have any questions, I'll be happy to answer them if I can.