s glorious steed Shadowfax
from the "Lord of the Rings
" flicks just took a permanent trip to the big glue shire in the sky -- after the horse got too sick and had to be put down.
Shadowfax's owner Cynthia says the horse -- real name Blanco -- suddenly fell ill at home and was rushed to a horse hospital ... but after a few days, things took a turn for the worst.
The veterinarian asked Cynthia if she wanted to put the horse to sleep ... so she asked Shadowfax for approval -- and the owner says the horse gave her a look that meant yes.
Cynthia gave vets the OK -- and they put her down.