E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
A horse of incredible swiftness, belonging to the four sons of Aymon. If only one of the sons mounted, the horse was of the ordinary size; but if all four mounted, his body became elongated to the requisite length. The name is used for any valuable or wonderful horse, and means a high-bay (bay-ard). (Villeneuve: Les Quatre-Filz Aymon.) (See HORSE.)
Keep Bayard in the stable, i.e. keep what is of value under lock and key. (See above.)
Bold as Blind Bayard. Foolhardy. If a blind horse leaps, the chance is he will fall into a ditch. Grose mentions the following expression, To ride bayard of ten toesGoing by the marrow-bone stagei.e. walking.