What was happening to the horses when they kept on working in the mud?

Chapter 12 depicts Joey and Topthorn's return to warfare. The experience is difficult, especially after the comforts of the farm, though there are other factors that make Joey's time pulling an artillery cart especially harsh, such as cold weather and mud. Joey and Topthorn also toil alongside a few other horses: a large and powerful horse named Heine, a foul-tempered horse named Coco, and two small, sturdy, lovable horses called the Haflingers.

The harsh conditions soon take a toll, sapping the horses of strength and killing Heine, the first of all the animals in the regiment to die. Coco is killed by flying shrapnel, and even the robust Topthorn begins to weaken. A doctor comes to inspect the horses; he warns the major who is in charge of the artillery unit to take better care of the remaining beasts, especially Topthorn. Unfortunately, the conditions that Joey and the other horses face—mud, hard work, and little food—do not improve.