I think it would be the Monarch s-gene

but i'm not 100% sure
edit:
I think it's in this order (most dominante s-gene first)
Clownfish--zebra--tarantula--monarch--fawn--- zebrafinsh
so if you breed two ponies who show fawn and zebrafinch. the baby will most likely show fawn.
(please correct me if i'm wrong)