The horse Willoughby intended to give Marianne was called Queen Mab.

Elinor overhears this rather intimate exchange between Marianne and Willoughby when Marianne is obliged to decline his gift: "But, Marianne, the horse is still yours, though you cannot use it now. I shall keep it only till you can claim it. When you leave Barton to form your own establishment in a more lasting home, Queen Mab shall receive you."

When Elinor hears this, Willoughby's tone, his use of Marianne's Christian name, the implication of what 'when you leave Barton to form your own establishment' suggests, and the intimate tone of the conversation convinces Elinor that they are engaged.

The meaning of the horse's name would not be lost on Marianne either. It is from a speech by Mercutio in Act I, scene iv of Romeo & JulietM:

O, then I see Queen Mab hath been with you.
She is the fairies' midwife, and she comes
In shape no bigger than an agate stone
On the forefinger of an alderman,
Drawn with a team of little atomies
Over men's noses as they lie asleep;
Her wagon spokes made of long spinners' legs,
The cover, of the wings of grasshoppers;
Her traces, of the smallest spider web;
Her collars, of the moonshine's wat'ry beams;
Her whip, of cricket's bone; the lash, of film;
Her wagoner, a small grey-coated gnat,
Not half so big as a round little worm
Pricked from the lazy finger of a maid;
Her chariot is an empty hazelnut,
Made by the joiner squirrel or old grub,
Time out o' mind the fairies' coachmakers.
And in this state she gallops night by night
Through lovers' brains, and then they dream of love;

All of the romantic fairy imagery and poetic language, and particularly the suggestion of the last line above would have had their effect on Marianne, to be sure. Marianne told Elinor that the horse "[Willoughby] had bred himself on his estate in Somersetshire" and that it "was exactly calculated to carry a woman." With a name like that, one wonders what else Queen Mab was 'exactly calculated' to do?

The first part of Mercutio's speech sounds romantic, but the second part, following the part about 'dreaming of love' is not. The second part has violent imagery, and suggestions of debauchery and women of ill repute, rather than love. It continues to speak of dreams, but rather than dreams of love, it is dreams of money and power over others.

Mercutio gets carried away by his speech and cannot stop himself--Romeo actually has to cut him off. In the same way, what starts out as a beautiful dream cannot be controlled or prevented from turning instead into a nightmare. The only way to stop the nightmare is to force the person to wake up.

It seems that Marianne sees nothing to disapprove of in what the name Queen Mab could suggest. Another instance of Marianne seeing what she wants to see?