That's a running horse weathervane. All those old early american images have loads of symbolism... Running horses are associated with fertility, empathy, travel, power, freedom, peace, sincerity, wealth and loyalty. They are referred in the bible and their tradition dates back to the Celtic mythology. In the bible, they are used to represent virtues like righteousness, holiness and purity and also stand for salvation. White horse weather vanes hold very strong symbolic messages. They are also particularly relevant for Irish people since their folklore and fairy tales consider white horses to be very majestic. Arabian horses represent peace, friendship, sincerity and honor.
Of course these guys may not even be aware of all that...maybe it's just there coz it's a horse and these two guys LOVE horses and it's odd and unusual to have it in the bathroom...

not to make too much out of it or anything

besides I like Jeff's idea, darn thing makes a great hand towel rack!