The pig icon is a throwback to Black Isle's Infinity Engine Dungeons and Dragons games - Baldur's Gate 1 and 2, Icewind Dale 1 and 2 and Planescape: Torment.
The spell icon in those was our little piggy!
I didn't know that. Thank you. Also, there is a tradition of coating a pig in grease and letting it loose at some event, where it runs about causing havoc and is very difficult to catch. A greased pig.