At Antietam the apparent organisation of the cavalry is:
Centre (Pleasonton's group)
5th US Cavalry
2nd Brigade (Col. Farnsworth): 8th Illinois (ref), 3rd Indiana (ref), 1st Massachusetts (ref) and 8th Pennsylvania Cavalry (no ref found for 8th PA)
3rd Brigade (Col. Rush): 4th and 6th Pennsylvania Cavalry (no refs found, but Pleasonton's report puts them here)
4x Horse Batteries
Right Flank (attached to 1st Corps)
5th Brigade (Col. Davis): 3rd Pennsylvania (ref) and 8th New York Cavalry(ref) (note, both histories agree they were on the right flank with Hooker, which is contradicted by Pleasonton's report, but supported by Hooker's report)*
15th Pennsylvania Cavalry (det, 200 men at most) (ref)
2nd New York Cavalry (2 sqns) (no ref, but was 1st Corps escort unit)
Left Flank (attached to 9th Corps)
6th New York Cavalry (ref)
The 6th US Cavalry was not on the field at any point during the 16th-18th, being in the Pleasant Valley attached to 6th Corps (ref)
In summary there were 14 major cavalry units on the field (excluding escort squadrons etc.). 7 were with Pleasonton, 6 were with Hooker and screened his advance. Only one was with on the left flank, the 6th New York Cavalry (only 4 sqns, one was detached as an escort). The opening shots of the battle were fired on the 16th by the 3rd squadron of the 3rd Pennsylvania Cavalry skirmishing in front of Hooker's Corps.
The history of the 6th NY is enlightening about the Battle of Antietam. They state that they did cross the Antietam and skirmished with Munford's brigade of cavalry, taking a few prisoners. Carman's map seems to place Whitting's regular cavalry brigade there too.
There is a lot of ambiguity here. No-one has done much research into the cavalry operations in this campaign, I suspect because even a casual inspection of the histories shows that the Federal cavalry totally dominated the Confederate cavalry on the field and this doesn't fit in with the required storyline (see Wittenberg for the current consensus). A lot more work needs doing, especially to find out what half of McClellan's cavalry were up to on the far right flank.
* Incidently, the 8th NY Cavalry was the only escapee unit from Harper's Ferry on the field.The 12th Illinois, 7th RI Sqn and 1st Md Sqn seem to have joined the army after the 17th.
PS: Many of the regimental histories describe the operations of the 14th-21st September 1862 as a single continuous fight. These continuous battles can be named by their conventions:
14th: Battle of South Mountain
15th: Battle of Boonsboro
16th: Battle of Sharpsburg Turnpike
16th-18th: Battle of Antietam
19th: Battle of Botoler's Ford
20th: Battle of Shephardstown
21st: Battle of Williamsport