1)
Was Yabetz mentioned before?
1.
Radak #1: Perhaps he is one of those mentioned above; he had two names.
2.
Radak #2: Indeed, he was not.
2)
What is the significance of naming him due to her pain in birth?
1.
'Rashi': This is like Rachel called her second son Ben Oni. Malbim - we can say that also Yabetz' mother died amidst giving birth to him.
2.
Radak: Even though Otzev and Yabetz are different (the letters are rearranged), there is no concern. The name Noach comes from Yenachamenu, Kayin from Kanisi, and Shmuel from me'Hashem She'altihu.
