mysteries of English etymology. Many expressions -- a dog's life (c.1600), go to the dogs (1610s), etc. -- reflect earlier hard use of the animals as hunting accessories, not pampered pets. In ancient times, "the dog" was the worst throw in dice (attested in Gk., L., and Skt., where the word for "the lucky player" was lit. "the dog-killer"), which plausibly explains the Gk. word for "danger," kindynas, which appears to be "play the dog." Slang meaning "ugly woman" is from 1930s; that of "sexually aggressive man" is from 1950s. Dog tag is from 1918. To dog-ear a book is from 1650s; dog-eared in extended sense of "worn, unkempt" is from 1894.
"Notwithstanding, as a dog hath a day, so may I perchance have time to declare it in deeds." [Queen Elizabeth, 1550]
"It is ill wakyng of a sleapyng dogge." [Heywood, 1562]
Phrase put on the dog "get dressed up" (1934) may refer back to the stiff stand-up shirt collars that in the 1890s were the height of male fashion (and were known as dog-collars at least from 1883), with reference to collars worn by dogs. The common Sp. word for "dog," perro, also is a mystery word of unknown origin, perhaps from Iberian.
"to track like a dog," 1510s, see dog