Since we pretty much figured out what Peggy is, why don't we try to figure out what Molly is.
Molly is the boss horse, I'm not sure what you call it, but all the other horses follow her orders. I don't know how exactly tall she is, idk how to measure in hands, but I'm about 5'4", maybe 5'5" and I'm shorter then the lowest part of her back. I have to stand on something to brush her butt lol. Her mane is super super long and her tail touches the ground. She's got her gutt sticking out a bit in these pictures, but when she's driving with Peggy she looks slimmer. She also an old lady, in her 20s.
This is her with Maude standing behind her. Maudes an average size horse.
I could try getting more pictures of her if needed. She looks so beatiful when she's driving with Peggy, she likes to hold her head in a kind of arch and picks her feet up real high :)