Aren't the small animals the Guosim (Guerilla Union of Shrews in Mossflower)? I seem to recall they have a very strong line in being scary and mysterious and frightening passers-by, and they're the kind of creatures who would wear skulls for helmets and make their own survivalist camoflage.

No, these are encountered by Matthias and company well into their southward pursuit of Slagar and the kidnapped children of Redwall—they have already met up with the Guosim by then. I don't think the text ever identifies them as anything other than "the painted ones." They kill several shrews and almost steal a young otter before our heroes get away from them; Slagar's party suffered similar losses when it passed through the same dark pines. They do not reappear in any of the books I read, although that doesn't preclude Jacques re-using them in a later novel, probably less effectively. In elementary school, because of the bones, I think partly because there never is a real explanation of who or even what they are, they bothered me.

The Redwall wiki tells me that they're a loving parody of the Liverpool longshoremen's union.

Well, I did not pick up on that as a child.

(not to post mystery links, this is Louisa Jo Killen singing "Bonny Bunch of Roses-O.")

Louisa Killen is always welcome in comments.