As per Mahabharata: Adi Parva: Sambhava Parva: Chapter 66, Valikhilyas were sons of Rishi Kratu:
It hath been already said, that Marichi, Angiras. Atri, Pulastya, Pulaha, and Kratu--these six great Rishis of great energy--are the sons of Brahman.
And the sons of Kratu, sacred as sacrifices, are the companions of Surya, (the Valikhilyas), known in three worlds and devoted to truth and vows.
Mahabharata: Adi Parva: Astika Parva: Chapter 30 mentions their ascetic penances hanging upside-down in the tree:
Sauti said, 'At the very touch by Garuda of great might with his feet, the branch of the tree broke as it was caught by Garuda. Casting his eyes around in wonder he saw Valakhilya Rishis hanging therefrom with heads downwards and engaged in ascetic penances. Reflecting that if that bough fell down, the Rishis would be slain, the mighty one held the elephant and the tortoise still more firmly with his claws. And from fear of slaying the Rishis and desire of saving them, held that bough in his beaks, and rose on his wings. The great Rishis were struck with wonder at the sight of that act of his which was beyond even the power of the gods, and gave that mighty bird a name. And they said, 'As this ranger of the skies rises on its wings bearing a heavy burden, let this foremost of birds having snakes for his food be called Garuda (bearer of heavy weight).'
This was kind of deadlock situation for Garuda so he kept on flying having elephant and tortoise in his claws and tree branch in heis beak. Later on at request of Kashyapa, Valikhilyas left that tree branch and went to Himlyas for their penance. Garuda threw that tree branch on a mountain, ate both the elephant and tortoise and proceeded to bring away Amrita by force from Indra to free his mother from Sarpas/Nagas.