First, you can get art.mul and artidx.mul by extracting the artLegacyMUL.uop file. Use UO Fiddler, UOPTool or LegacyMULConverter-N. Although art files are not needed to create animations at all.
Animation - there are several programs how to work with them. You can use mulpatcher, UO Fiddler, or an older UO anim tool. Basically, it needs to produce individual frame images for animation, which you then have to wrap in .vd format and insert into the anim.mul file (or anim2, anim3, anim4, and anim5). All you need is a graphics program, mulpatcher and UO Fiddler. UO Fiddler replaces the collision program UO anim tool for positioning individual frames.