He had a butterfly way back when. Said butterfly evolved like multiple times. During the Ancient Battlefield arc, said butterfly had evolved into some Yin/Yang shit with little Yin/Yang energy gathered at her antennas (two antennas, one each). The Yin energy antenna gathered the energy at the Ancient Battlefield and gave birth to Lil' Black. That's my take on it. I'm pretty sure that's what happened, the author was just sparse on the details.
Lil black is ling worm[4th grade] used to find ling vein originally Tenth Grade is that beast lover best work before coming to turtle island Lil'black [still faithful son] seniority within Ge family hierarchy one rank below silly bird [rebellious big sis], he is 2nd followed by lil'pagoda [greedy son] then lil'fire[playful one]
Lil'black is the ling worm that he raised at the wukong sect and it's used to search for a ling vein [Like a treasure radar]