Ramayana
दृष्ट्वा देवास्तु तद्रक्षो वरदानेन दुर्जयम् । तिर्यग्योनिं समाविष्टास्तस्य दर्शनभीरवः ॥ 7-18-4 ॥
But the gods, seeing that Rakshasa made invincible by the boon granted to him, and fearful at the sight of him, entered into the wombs of animals.
english translation