I would say get rid of the message completely.
The notification is nice for those who want it, some people like to keep all their pound rescues.
So, if it were to stay, the message should be changed to just another notification such as what's included in the poll, but maybe add in something heartfelt for those who want that. Such as:
"[Pet] you included in a trade is a pound rescue!
Cancel/Edit if you wish for them to stay, or allow them to find another good home!"
It's informative of the Pound Pet, and it represents the pet in a better condition, but is allowing of the pet to venture beyond their rescue home. You have the choice to have them stay, or "allow" them to go on.