I'm inclined to think that 'Queen Mab' does exist, though with a name which can be altered to suit circumstances.

And when I'm feeling suspicious of Willoughby's motives, it occurs to me that this horse is exactly calculated to carry a woman, and is therefore not suitable for Willoughby to ride. Furthermore, this horse is currently eating her head off at Combe Magna, an expense which W can ill afford.

He has been trying to persuade Brandon to buy his brown mare. So it may have occurred to him that giving Queen Mab to Marianne would kill two birds with one stone. The gift of the horse might help him get the girl, whilst at the same time relieving him of the cost of the animal's upkeep.