Granted, the first mutation that would be needed would probably be one resulting in a Coelurosauravus like creature, though that's not too hard to imagine - indeed, it may be that such a trait could be bred into them.
Creating a Dragonet may not be all that difficult. Would it not bring much profit to my lab because of pet sales? :p
Of course, such creatures may already exist -there is much we've yet to find. Say, tales of exotic places reach Medieval Europe, with flying lizards which have wingspans of 30... centimetres, not metres, but don't tell them that... which were slain by our brave knights... for added effect, add in something about their flaming hot breath.
