Notice that the female rider is held at the waist by the armoured man at her side. At these times the sidesaddle wasn’t yet entirely suited for individual riding - the lady’s pose itself suggests she probably had a poor control of her horse. Indeed the first “sidesaddle” - according to wikipedia - was used by Anne of Bohemia in the late 14th century and was more of a chair fitted on horse’s back, allowing the woman to sit more or less securely while being led around, facing the side and having minimal control of her steed herself.
This lady’s dress is interesting because it combines the characteristic features of the Burgundian v-neck gown - the tight sleeves, the high belt, the overturned cuffs and the hem displaying the white fur lining - but with squared neckline, partially covered by a sheer partlet.
The gown of the kneeling woman seems to be a variation on houppelande, with the collar turned down. Elements specific for the style and visible here are for example the full front, pleated when constricted by a customary wide belt (here obscured by the wearer’s hands) and the voluminous sleeves, in various styles, but generally in stark contrast with the tight, sleek sleeves of the later Burgundian v-neck gown.
Check out the black dragon-horse in the background!
Also, this image is an example of attempts at depicting “historical clothing” in the 15th century - or of artists taking considerable liberties with the interpretation of those, more like. Most often these seems to be based on contemporary clothing, but with quirky twists. For example, the blue dress of the woman with the red undersleeves has what seems to be a golden fringe decorating its short sleeves, obviously a fanciful touch.