Pink Tentacle reports:
This suit of dog armor — identified by antique Japanese armor dealer Toraba.Com as the only known and certified authentic example of its kind — is believed to have been created for the pet of a wealthy, high-ranking and presumably eccentric samurai or daimyo (feudal lord) in the mid to late Edo period (mid-18th to mid-19th century).
Spencer Burns at Yachigusa Ryu wonders why this innovation didn't occur during the fifth Tokugawa shogunate, when Tokugawa Tsunayoshi (1646-1709) "prohibited the harming of dogs and ordered the establishment of dog kennels, [thus earning] the nickname of 'dog shogun.'"
http://www.hrgiger.com/missing/WorkNo075b.jpg
They did not actually enter the battle unless the enemy lost face by retreating. The Akita dog has twice the bite pressure of a German Shephard, three times that of a Doberman.
One prominent Damiyo kept a kennel of 1200 war dogs, each with its own handler.
But the armor shown above would have probably been cerimonial, the helmet would have made biting impossible and the backpiece would have slowed them down.