mantle, capelet and shoulder_cape are all used to tag a short cape-like outer garment that falls to about elbow-height.
capelet was adopted in forum #13654, where a possible mantle/capelet alias was also brought up (but no conclusion was reached). shoulder_cape came into use recently, mostly tagged by denim332.
Curiously, mantle is predominantly of the santa_costume flavor -- which may justify keeping it distinct. But the other two can probably be merged one way or another.
Updated by albert