Thick rubber, have held up to 24/7 use so far (about 2 weeks) without issue. They do run a little bit small. I got an XL for my size 2 shoe warmblood and fits okay, but could be a little bit longer and a wider opening. These were a little challenging to put on and take off, but not the worst I've experienced.
Centaur Economy Heavy-Duty Ribbed Bell Boot available at HorseLoverZ, the #1 place for horse products and equipment.
Rubber composite pull-on boot.
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