Was Wheel Horse the only one to have these things?
And what do you do with them?
Do they make plows and whatnot for them?
1) Wheel Horse was the only one to use that style of slot hitch, but several other manufacturers had their own version of it.
2) You hitch things to a horse.
3) Wheel Horse, in 1965, had roughly 25 attachments for their tractors, many of which attached via the slot hitch. Plows, cultivators, disks, seeders, spreaders, carts, wagons, tillers, rakes, blades, and the list goes on.
Here's a link to the Wheel Horse Interchangeability List. Look up 1955-1965 and take a gander at how many attachments their were.
Also, here's an example of what WH was making as early as 1955. The example is for a 1958 RJ-58 advert.