Looking at it from a different perspective, I do think it is possible that Willoughby is on the level here:

He is smitten by Marianne, wants to please her, and he has a spare horse at home.
His gift of the horse is an impetuous act ( "there was an eagerness in everything he did...").
It is a thoughtless act (he hasn't thought of the expense to the Dashwood ladies, and his continually increasing debts suggest that he isn't in the habit of thinking prudently before he spends money).
But it is not a consciously manipulative or Machiavellian act.

And suppose he did change the name to appeal to Marianne's romantic sensibilities - well so what? Unless Queen Mab has been trained to answer to her name, it won't make any difference to the horse.

My point is that I think Willoughby's actions, as they appear at this stage in the novel, are capable of favourable and unfavourable interpretation. To a first-time reader the situation is ambiguous at this point, and I believe that was precisely JA's intention.