The substance discovered after testing on cats that prevents the clotting of blood is an anticoagulant. More specifically, anticoagulants are medications or substances that prevent the formation of blood clots. This is supported by the context indicating anticoagulants are used in veterinary medicine, including for cats, to inhibit blood clotting. Historical discoveries related to blood clot prevention involved substances like sodium citrate, which was found in early 1900s animal experiments (including dogs) to prevent clotting of blood.

To summarize, the answer is anticoagulants —a class of agents discovered and tested in animals like cats that prevent blood clotting. If the question is from a multiple-choice list referencing these terms: antibiotics, antiblood, anticoagulants, anesthesia—the correct choice would be anticoagulants since they directly prevent clotting. This matches well with both historical research and veterinary use for clot prevention discovered in animal testing.