… the Cappa Magna originally was a horse blanket?
When cardinals rode on horses or – in the early middle ages – on mules (Matthew, 21), they protected the animals against the cold by having their capes made wider, so not only the men, but also the mules were covered. I don’t exactly know when horses started to become 30 feet long, though. Nah, just kidding. The lengthening of the Cappa is, of course, due to the Church’s ability to see something good and make it better.
Here is an excerpt from the “Stanza di Eliodoro” by Raphael. It shows Leo the Great confronting Attila. The confrontation ended with the retreat of the Huns after, with the assistance of God (and of the saints Peter and Paul, which we see above the Pope), Leo convinced Attila that no good would come out of attacking and sacking the defenseless city. If we take a look at Greece, we can only imagine what would have happened to Rome, had the Huns indeed attacked and devastated it.
The face of the Pope is a portrait of Leo X, successor of Julius II, Raphael’s original patron.
Behind the Pope you can see two cardinals on horseback. Their capes are covering the horses as well. There you have an early form of the Cappa Magna. During the time of Leo X. long trains were already in use, though.