I did a pattern for a C15th-C16th pair of leather mittens found in the Thames shoreline and now in the Museum of London for the the Companie of St George Dragon a few years back .. not sure if it's online yet. However I think the Medieval tailors Assistant has used the same pattern.They thumb is made from waste when you shape the top of the glove if you're careful, the main point is that the cut for the thumb in the palm section has a V left which forms the inside part of the thumb itself.
The Thames mitten is heavilly worn on the palm and looks like cow hide, though thin, but that's just by observation not analysis. The stitching was neat butt stitched from recollection.
The Shepherds 2-finger mitten in Campin's nativity appears to be sheepskin.
Traditionally whitetawed (alum tawed rather than tanned) leather was preferred for gloves and traditionally non bovine (e.g. deer, goat, sheep) was tawed.
Hope that helps
Cheers
Dave