The Boeing 727QC was the first, and for a long time the mainstream, type to have palletised seats. This was a fully certified original manufacturer's option, not a subsequent mod, and was a very popular buy in the USA with mainstream carriers in the late 1960s. In those pre-FedEx/UPS days it was common for major US carriers to have an extensive overnight scheduled freight network, the seats were stripped after the last passenger flight of the day and reinstalled into another aircraft arriving early the next morning.
Seats came in and out on forklifts through the front cargo door. Took about an hour to change the config. Many of the 727s that FedEx and UPS later operated with were originally these aircraft. The last users of such aircraft this way were UPS who had some weekend pax charters down to the Caribbean when freight business was reduced, so they put the seats in and out once a week rather than daily, and used contract flight attendants. UPS only gave up such passenger charters a couple of years ago, when their convertible 727s (still the original late 1960s ones) were finally retired. It's better suited to the package operators as the overhead PSUs and bins obviously have to stay in place during the cargo ops, so an interesting challenge if you are going to be carrying horses, which others will be better placed to describe.