I recall reading when I was researching the history of my regiment, there was mention in Ensign William Leeke's memoirs that most of the regimental officers were mounted on horseback at Waterloo. How it happened after loosing their prior horses, was stealing the stray horses in the battlefield.
I remember reading that the Colonel himself had his horse shot under him and was wounded by another bullet as well in the progress. Another colonel of the regiment also lost his horse which led to an intense horse hunt.
The regiment was classified as light infantry as well, so I'd see it being preferable back then to have the officers being mounted from the leadership's perspective.
Edit: I found the following about horses in William Leeke's memoirs "The history of Lord Seaton's regiment at the battle of Waterloo" (1866), page 7
Two or three days after I arrived Sir John Colborne joined from Brussels and took the command of the regiment I had only seen him once before and stood in considerable awe of him though I was thoroughly convinced that he had the kindest feeling towards me He was a sort of nephew of my mother's her brother having married his mother
He told me I remember that he thought from Sir James Kempt's description of the person he had met in the canal boat that I had arrived out. He recommended me to purchase a riding horse in addition to the one I had for my baggage. I told him I had been intending to try and do without a horse until after the first action in which no doubt many officers would be killed and then a horse would be purchased at a cheap rate, he smiled I suppose at my warlike and sanguinary ideas and merely said: "You had better get a horse at once". This I did and purchased a black horse with a long tail very much like those used by the "Life Guards and Blues"
Sir John Colborne always strongly advocated the importance of infantry officers when on active service having riding horses and used to say that if from insufficiency of income they found it difficult to manage this still they should stint themselves in wine and in every thing else in order to keep a horse if possible.
As mounted officers they were more useful under very many circumstances they were less tired at the end of a day's march and more ready for any duty which might be required of them they could be more effective in bringing up stragglers on a long and weary march some of them might be usefully employed when extra staff officers were required. I think on the long march of upwards of fifty miles which we had from Quevres au camps to Waterloo all but two of the officers of the 52nd were mounted.
During the five weeks between my joining at Lessines and our start for Waterloo I went through some portion of my drill which soon after our arrival at Paris was completed in the Champs Elysees