Metamorphs

Poul Anderson presents were-beasts, including even scientifically rationalized werewolves, in works of fantasy but I think that he stops short of presenting any "general purposes" metamorphs that would be able to assume any shape whatsoever at least within their size range?

I ask this question because:

James Blish does give us one such "general purposes" metamorph (see here);

I have just been reading about eight-foot reptilian shape-changers infiltrating Earth in a Marvel comic, The Ultimates;

however, all-purposes metamorphs seem particularly implausible and therefore I would expect Anderson to avoid them.