I read the first book many years before the TV show and all this subsequent information from <PERSON>'s histories like Fire and Blood written so many years after the original novel, so my take is based on the dialogue in the first book and having read a zillion fantasy books.
The Maegi seemed to have used a spell that exchanged life for life, which was the purpose of the stallion for <PERSON> but the baby's life was also drained but mostly into the Dragon eggs, which were fossilized and hundreds of years old. I always interpreted the line about <PERSON> being "dead for years" as to reflect a direct exchange of the actual condition of the dragons prespell into <PERSON> and his life into them as well as some of their appearance. I also think that without the spell, he was a normal child, because <PERSON> later has a vision of him as a tall bronze man with white hair braided with the Dothraki and he is normal in appearance there and in the vision in the House of Undying, again he is normal in that one, and was in the TV adaptation in that scene. The <PERSON> in the book version is very glad to gloat over killing <PERSON> and that "the stallion that mounts the world" prediction by the <PERSON> of <PERSON> will not come true at all. She has made sure no son of <PERSON> will rule the world.
So, I think poor <PERSON>'s appearance was a mess just like stated, but because of the spell rather than some random Tarygaren birth defect. 247
All that stuff was written years after the first book in the Histories by <PERSON>, and may have been added to make <PERSON>'s death more mysterious and to bolster stronger ties of the Tarygarens with the Dragons and his hinted Valyrian hybridization rituals. He also has admitted experimenting with past characters or concepts to please fans or add more interest to what has been already stated, <PERSON> is an example of <PERSON> trying to recreate the excitement the Red viper had on his fanbase by his own admission to fans. He also was interviewed and said he was shocked so many people figured out <PERSON> and <PERSON> were <PERSON>'s parents, and he liked to keep more things obscure than that, and now he has added a Tarygaren Princess who accidently died of Fireworms from a flight to Valyria in Fire and Blood. (Although R+L=J was easy to see in the FIRST book to me!)
So I think the recurrent birth defect like dragon thing is something retconned into GOT, although stillbirths and defects could occur easily in an inbred family like that of course. FIrst book, it seemed the spell caused it, period... 247