my impressions could be wrong
calf - supple/medium weightish/more durable than lamb, most common or second most common after lamb
donkey - durable/tough like horse but maybe a bit less durable and a bit less thick/more grain (smaller animal), not many people produce it/use it
horse - durable, generally thicker, very scratch/dimple resistant... some scratches will just fade as the leather undimples. can be soft or stiff... wide variety of types depending on part of horse...