Others probably spotted it but I only just realised that the Lions that form part of the final version of the traction city of London, where actually there in the original concept art that Peter Jackson released:


That object in the yellow circle is totally the back of a lion.

Right?

If you need a reminder, here are the lions from the trailer:


So it would seem that director Christian Rivers settled on the Lions as part of the whole shebang pretty early on, though of course they could have been described in the script...