(Spoilers Extended) Who is the 8th bastard of Robert I Baratheon that Varys knows of?

According to Maggy the Frog, Robert had 16 children:

Anger flashed across the child's face. "If she tries I will have my brother kill her." Even then she would not stop, willful child as she was. She still had one more question due her, one more glimpse into her life to come. "Will the king and I have children?" she asked. "Oh, aye. Six-and-ten for him, and three for you." That made no sense to Cersei. Her thumb was throbbing where she'd cut it, and her blood was dripping on the carpet. How could that be? she wanted to ask, but she was done with her questions. -AFFC, Cersei VIII

Varys knew of 8 of these 16:

"He fathered eight, to the best of my knowing," Varys said as he wrestled with the saddle. "Their mothers were copper and honey, chestnut and butter, yet the babes were all black as ravens . . . and as ill-omened, it would seem. So when Joffrey, Myrcella, and Tommen slid out between your sister's thighs, each as golden as the sun, the truth was not hard to glimpse." -ACOK, Tyrion III

We know 7 of these 8:

1)Gendry

2)Mya Stone

3)Bella

4)Edric Storm

5)Barra

6/7)Twins fathered on a serving wench at Casterly Rock that Cersei has killed

So there is 1 bastard that Varys knows of, that we do not know the name of yet. Does anyone have any ideas/ theories on who #8 might be?