From the ArcaMax Publishing, Cats & Dogs Newsletter:
http://www.arcamax.com/news/catsanddogs/s-173661-100769
Rex was a Cavalier King Charles Spaniel owned by Nancy and Ronald
Reagan while the latter served as President of the United States. Rex
had belonged as a puppy to conservative commentator William F.
Buckley, Jr. who gave political ally Reagan Rex as a Christmas present
in December of 1985.
Rex would live in the White House from that Christmas until Reagan
left office in 1989, once gaining headlines when he underwent a
tonsillectomy. He was apparently named for Rex Scouten, the White
House chief usher who retired in 1985. Rex was treated to a lavishly
decorated doghouse, which included framed portraits of Ronald and
Nancy and red window draperies.
After Reagan left office, Rex lived to the age of 13 before being
euthanized after developing an enlarged heart. He was 86 in dog years.