Yes, those nobles with their multitude of titles!
Meinhardt Schomberg, the eldest son of Frederick Schomberg, was created Duke of Leinster in 1691. This was a reward for his services in Ireland (Boyne and Limerick).
Charles Schomberg, the younger son of Frederick Schomberg, succeeded his father as 2nd Duke of Schomberg after the latter was killed at the Boyne in 1690. Charles later commanded troops in Piedmont that were paid by England/Dutch Republic, and was killed at the battle of Marsaglia in October 1693. Meinhardt then succeeded as the 3rd Duke of Schomberg.
So indeed, at the time of Neerwinden, a bit before Marsaglia, the future 7th DG could be found listed as Schomberg's Horse (if correct without any Duke), or as Leinster's Horse (with or without Duke).
As to the regiment itself, that came into existence in November and December 1688. Initially it were simply troops of horse were raised in Derbyshire, Nottinghamshire and Leicestershire by the 4th Earl of Devonshire (future 1st Duke). I have not studied this person in much detail, but I think he had much influence in those counties, and the troops were probably levied from his clientele and their servants. The initial troops would most likely have looked like a rabble of armed men, than anything regular
The troops were placed on the regular establishment and regimented in 31 December 1688. The regiment was given to Lord Cavendish, the son of the 1st Duke and future 2nd Duke.