An absolutely massive snake would work as a daemon for the 5th duke — snakes are often associated with intelligence and in the regency were romantic symbols, but at the same time they’re lethargic and solitary. A fang-less python large enough to be encumbered by its own weight and appetite would sum up a powerful-and-intelligent but cold and lazy man quite nicely.
Georgiana C would presumably have a leopard daemon, to reflect her hard-partying ways, and would be called ‘the leader of the whiggish maenads’ in the press (or something), leopards being sacred to Bacchus. (See also: Lord Byron)
Bess Foster would have a large smug domestic cat with loads of hair.
Young Hart, I’m inclined to think, would have a daemon always changing into something with less than five senses — moles and so on. Or, possibly, animals with very keen hearing, but in a way that he couldn’t translate into his own use and understanding.