They can ride a horned one instead. But it's a magic item (around 30-35 pts), so it eats from the macig item points that you are allowed.
Cold ones are a bit too big for skinks. I don't think horned ones should have been made magic items, they should have just been mount options, but GW does what they wish...
I agree with strewart. I have no idea why they have become magical unique items, if they were going to be made into enchanted cold ones or some such, they should have been made as a more than one type of item.