The one we've used a couple of times on group rides is to do the 'slow down' wave followed by holding your hand up to the side of your helmet with the index finger pointing up and out, with the thumb pointing in a different direction (think antlers or horns). Also, sometimes we point in the general direction we saw said deer or cow if there is sufficient warning, to let those behind us know about it. Kind of like pointing your toe at debris you are swerving to avoid.
This is by no means a standard hand signal, but it conveys the message to approaching riders about livestock or wildlife well enough.