Horses, mules, dogs, pigeons, cats, camels, koalas, even elephants were enlisted for their service in World War I. While a few were just mascots for the purpose of morale, most were put to work carrying soldiers and equipment. The British dog pictured here was photographed around 1915, delivering medical supplies to the front lines. The Atlantic has a collection of 45 photographs of animals in wartime. Warning: while not overly graphic, a few images contain deceased men or horses. -via Everlasting Blort
"Cher Ami flew with bullets zipping through the air all around her. Cher Ami was eventually shot down but managed to take flight again. She arrived back at her loft at division headquarters 25 miles to the rear in just 25 minutes, helping to save the lives of the 194 survivors. In this last mission, Cher Ami delivered the message despite having been shot through the breast, blinded in one eye, covered in blood and with a leg hanging only by a tendon. The pigeon was awarded the Croix de Guerre Medal with a palm Oak Leaf Cluster for her heroic service in delivering 12 important messages in Verdun. To American school children of the 1920s and 1930s, Cher Ami was as well known as any human World War I heroes. Cher Ami's body was mounted and enshrined in the Smithsonian Institution."
It's a great film closely based on actually events. In many ways, it is essentially a prequel to Band of Brothers, but which only few people have even heard of.
The story got some coverage here a few years ago:
http://www.neatorama.com/2012/05/28/the-lost-battalion/