User blog comment:Osoguineapig/Hounds Info Hunting!/@comment-88.108.195.242-20121201134657/@comment-5816499-20121204233557

they will attack Treeguards as long as they are already spawned on the field, they will attack it whether it is on active or passive state, they will also attack butterflies, i'm not sure with birds but i guess they can flee before the hound reaches them, haven't seen them attack toads yet but i'm pretty sure they will too, pigs and werepigs are aggresive to any other aggresive entity and will try to attack hounds even if the aggro is not on them turning the hound's aggro on them in the process, haven't seen them with the swamp monster yet though...

based on observation, the condition for aggro are: anything that is alive and nearest to the hound upon spawning will attract the hounds aggro, BUT, any kind of meat on the ground between the first target and the hound will attract it and will cause the hound to eat before attacking, aggro will reset whenever the hound stops (stops for an attack or stops to eat) and will retarget to the nearest entity