Sheep wool has traditionally been viewed as the representative mammalian keratin fiber for the purposes of describing morphology and protein composition. We have investigated narrow fibers from the under-hairs of a range of species both closely and distantly related to sheep, comparing structure and protein composition. Within this group, curvature was negatively correlated with diameter for all but mohair. The cortical cell types present in alpaca, rabbit, and mohair fibers differed structurally from wool, primarily in terms of their macrofibril architecture. Except for rabbit, each species' fibers contained three cell types, and except for mohair, cell types were distributed asymmetrically across the cortex. In mohair, the cell types were distributed annularly, and each cell type had regions in which intermediate filaments were packed into highly aligned hexagonal mosaics, much like the mesocortex in wool. Coupled with this, were differences in the protein profiles; the rabbit fiber contained extra keratins and keratin associated proteins, while only subtle differences were noted between mohair and Merino fibers. In both rabbit and mohair fibers, the relative abundance of keratin K85 was lower than that of Merino. These results suggest that there may be links between relative protein composition and fiber morphology, albeit complex ones.