http://www.wizards.com/dnd/3E_Monster_0800.asp
"There's been this little secret in the heart of the D&D game for many years. A group of the most powerful creatures in the game (the dragons) have the ability to polymorph -- either into human form, or essentially at will into any form. This allows the dragons to walk among the people of the land undetected. And some dragons, like the old Greek gods, are a randy lot indeed!
...
Dragons’ magical nature allows them to crossbreed with virtually any creature. This usually occurs while the dragon has changed its shape; it then abandons the crossbreed young.
