What were the first animals to be successfully vaccinated against the West Nile Virus?

Answer 1

At this time, "probably successful" seems to be the best claim; there has been insufficient time for an extended test.

According to
Samina I et al. An inactivated West Nile virus vaccine for domestic geese-efficacy study and a summary of 4 years of field application. _Vaccine_. 23(41):4955-8, 2005 Sep 30.
-- They used domestic geese in 2000, with 85% protection.

An earlier study,
Lustig S et al. A live attenuated West Nile virus strain as a potential veterinary vaccine. Viral Immunology. 13(4):401-10, 2000.
-- They use mice and geese in probably 1999, but no follow-up studies have confirmed positive results.

Answer 2

horses