It’s intended to work the way Fourdottwoone described with only one pet image showing on any particular bonus box.
It was designed this way to future proof. We currently have over 1000 Troops, as time goes on who can tell how many Pets there will be in the game and it would be unweildy to show them all in the same bonus box or give them all separate boxes.
I’ve updated the thread title for the bug report linked above as well.