Anything you consider to be abusive to the animal. For example. Sometimes it is quite obvious, the dog growls at the people, they are holding him by tail, legs, is bound or muzzled. The penis is handled improperly like sucking a limp penis or preventing it from going back into the sheath. Sometimes the dog is laying on his back and struggling to keep erection while someone is sucking him. Human like sucking and shaft rubbing, dildogging is also considered uncomfortable. Sometimes you will notice the dog not liking it. The same applies to horses but someone more experienced with them could give you better advice. Fake cum is not against the rules but a good indicator that something else might be wrong too.
Always provide a description of what is wrong to the mods in the report and comment and explain the mistakes in the thread so other users can reflect on their liking of the video. Most of the time it does nothing, but sometimes some exceptional user will start an argument with you that dog cum is a thick gooey, glue like liquid and you know nothing about dogs.