Before the emergence in the second half of the 18th c. of veterinary medicine as an academic discipline, the trade most involved with the care of domestic animals was that of farriers, who shoed horses and along the way acquired basic knowledge of equine injury and disease. The word veterinarian, however, from Latin veterinarius, ‘person who treats diseases of working animals/cattle’, predates the academic profession, first appearing in a 1646 work of Sir Thomas Browne.
The first institute of veterinary medicine was founded in Lyon in 1762; its graduates were called, as they are today, vétérinaires, a likely source for the alternative English veterinary. This school was soon followed by others in Vienna and Padova, 1765; Göttingen, 1771, and finally, London in 1790. The London college was headed by the Frenchman Benoit Vial de St. Bel, who may have given a further boost to nominal veterinary.
The earliest example from the UK I could find dates from 1819:
In Flanders it is effected by the practice of a similar system to that which was first introduced into England, by the useful publication of Strickland Freeman, Esq. and which has been since adopted, and pretty generally circulated through the united kingdom [Belgium], by the works and practice of Mr. Colman and other veterinaries… — Thomas Radcliff, A Report on the Agriculture of Eastern and Western Flanders, London,1819, 220.
Querying for the plural conveniently eliminates any attributive use. Although Merriam-Webster suggests an 1861 appearance in North American English of nominal veterinary, the word is actually attested some decades earlier in an academic journal:
But this very success made Priesnitz many enemies; the priests cast anathemas upon his devilish art: the physicians and veterinaries denounced him for practicing illegally, and the authorities were obliged to intervene. The Western Journal of the Medical and Physical Sciences, vol. 12, no. 45, Cincinnati OH, 1838, 134.
Veterinary as noun, however, has never enlarged its role as a seldom used alternative to veterinarian, a fact reputable dictionaries should have noted in their definitions. Of those American newspapers accessible through elephind.com, veterinaries appears 1,015 times compared to 32,179 hits for veterinarians. The much more current iWeb Corpus, now at 14 billion words, shows an even more radical disparity in frequency: veterinaries, 19; veterinarians, 27,920. Unfortunately, this corpus does not produce search results with a unique URL, so I can provide no links.
Infrequent though it may be, the word still finds use on both sides of the Atlantic:
Before the treatment begins, a veterinarian who supervises the whole procedure must immobilize the rhino. Hence, one member of the otherwise changing RRP task force is always a veterinarian. South African veterinaries use etorphine, which is better known as “M99”. — Martin Angler, Scientific American Blog, 9 May 2013.
Here, nominal veterinary avoids using veterinarian/s three times in so many sentences. The following article from a small town Texas weekly also uses vet and veterinarian several times, so nominal veterinary can avoid too much repetition here as well:
Last year 1,497 vaccines were administered to dogs and cats at the rabies clinics. The Cherokee Animal Clinic gave 604 shots, the most in the county. The two veterinaries there are Dr. Anthony Holcomb and Dr. Justin Novak... — The Cherokeean (Rusk TX), 7 May 2008.
A Virginia newpapers uses the word to mean ‘veterinary practice’ or the place of business of a veterinarian:
The Northern Neck Humane Society will have Holiday Angel Wreaths at businesses and veterinaries throughout the Northern Neck over the Thanksgiving and Christmas holidays. Parents can place the names of children, pets or loved ones on the wreaths. — Rappahannock Record (VA), 18 Nov. 2004.
By the end of the 20th c., the word had not completely vanished from UK English, here making an appearance in a debate in the House of Lords:
British veterinaries are of an extremely high standard. The degree courses are monitored by the Royal College of Veterinary Surgeons under the 1966 Act. — The Parliamentary Debates (House of Lords), 1988.
Even if someone were reading or hearing nominal veterinary for the first time in these sources, the meaning is immediately clear, but the economy of one syllable was not enough for the word to compete with the older and far more frequent veterinarian.