Might very well be a dragoon insignia - The 15th Penn Cav was a unique group all around, initially known as The Anderson Troop, they were recruited as bodyguards/escorts from upper echelons of Philadelphia society. They were a well dressed, dandy bunch

. Their distinctive uniform jackets were part of their esprit de corps, and their early and varied headgear were part of it too. You see pics of all manner of caps and hats in early war images with various adaptations of hat brass, insignia, company letters, plumes, etc.
Once they went to full regimental strength in the summer/fall of '62 things changed a little - they stuck with the cool jackets well into late '64 (and possibly longer), ditched their dark blue trousers and the headgear became more standardized with forge caps and Hardee/slouches being dominant in images.