I made these IMMs mostly for fun, but I thought other people might get some use out of them too. The first IMM brush includes 16 protofeathers, which according to my research tended to be more simple than modern feathers and usually were symmetrical. I've included three spikes in this set which have UVs already made for ease of use. The second IMM set is a fur or "dino fuzz" brush with 13 meshes. These can be used to quickly create convincing fur in ZBrush without having to rely on fibermesh. The geometry created is good if you need to bake it down to a low poly mesh, or if you are concepting.
Two IMM brush sets with a total of 29 unique meshes for ZBrush 2020.1 and up.
OBJs for users of older ZBrush versions. Step-by-step instructions for creating your own brush are included.