Actually the dragons just have alternate form. It specifies that they keep their type in the alternate form description.<br/><br/>"The creature retains the type and subtype of its original form. It gains the size of its new form. If the new form has the aquatic subtype, the creature gains that subtype as well."<br/><br/>I would assume this means that a dragon who uses its alternate form would make a half-dragon, but one that uses polymorph would be treated as the species it turns into.