A prone creature, a common sign of overcrowding.
Overcrowding occurs when too many tame animals share the same tile. Overcrowded animals may become enraged and start fights with other animals or passing dwarves. While large animals may be dangerous, even the meanest critter can incessantly spam the combat reports log. To avoid overcrowding, groups of animals should not be confined to small spaces, such as ill-devised pastures, small meeting halls, or pits. For non-grazing creatures, a cage can easily hold any number of animals without overcrowding. Grazers generally require a pasture, however multiple creatures in the same pasture tend to cluster in the upper-left corner, potentially leading to overcrowding despite sufficient pastureland. Each animal can be assigned to its own pasture to prevent overcrowding, at the expense of space and significant micromanagement. Interestingly, swimming creatures do not appear to experience overcrowding. Adding water to a containment pit/pond at a depth of 4/7 will curtail fighting, though it does make the animals more difficult to recover.
Overcrowding can be used to gradually train all your dwarven children in fighting. Designate a small zone as a pen for your war dogs and as a meeting zone. Your idling children will hang out with all the dogs, who will gradually attack them (and each other), giving everyone slight combat experience (and probably many, many, many scars).