Those sound like the rubber slip on bell boots :) We used to use them for Skippy! Because he would tear his other bellboots off. He couldnt do that with these though! LOL! They are a pain to get on and off, you have to stretch the living bajeezus out of em to get them over the hoof.
One day, Skippy! Pulled one of his over his kneecap.. LOL only Skippy! Could pull that off! LOL!
My general rule for riding is, return the horse to its stall/pasture exactly how it came out. So if you brought the horse out with bell boots and a fly sheet on, then return it with the bell boots and fly sheet on =)
By the way, Bell boots are rubber bell shaped things that completely cover the hoof/coronet band in most cases. Primarially, they are used for horses that over-reach when they walk/trot and have a habit of pulling their front shoes off with their over-reaching back feet. They are also used a lot for a similar problem: injury/damage to the front feet/coronet band when the back feet over reach and clip them.
Velcro ones http://www.moonrakerqh.com/tack/gfx/rubber-bell.jpg
Slip ons (the ones I believe your horse was wearing) http://www.stoneycreektack.com/image..._turn_bell.jpg
No turn bell boots that velcro in the front, and keep the back of the hoof completely covered.
I hope my post wasnt too confusing, LOL im sorta wonky today