SB 5.26.11

ye tv iha yathaivāmunā vihiṁsitā jantavaḥ paratra yama-yātanām upagataṁ ta eva ruravo bhūtvā tathā tam eva vihiṁsanti tasmād rauravam ity āhū rurur iti sarpād ati-krūra-sattvasyāpadeśaḥ.

ye—those who; tu—but; iha—in this life; yathā—as much as; eva—certainly; amunā—by him; vihiṁsitāḥ—who were hurt; jantavaḥ—the living entities; paratra—in the next life; yama-yātanām upagatam—being subjected to miserable conditions by Yamarāja; te—those living entities; eva—indeed; ruravaḥ—rurus (a kind of envious animal); bhūtvā—becoming; tathā—that much; tam—him; eva—certainly; vihiṁsanti—they hurt; tasmāt—because of this; rauravam—Raurava; iti—thus; āhuḥ—learned scholars say; ruruḥ—the animal known as ruru; iti—thus; sarpāt—than the snake; ati-krūra—much more cruel and envious; sattvasya—of the entity; apadeśaḥ—the name.

In this life, an envious person commits violent acts against many living entities. Therefore after his death, when he is taken to hell by Yamarāja, those living entities who were hurt by him appear as animals called rurus to inflict very severe pain upon him. Learned scholars call this hell Raurava. Not generally seen in this world, the ruru is more envious than a snake.

According to Śrīdhara Svāmī, the ruru is also known as the bhāra-śṛṅga (ati-krūrasya bhāra-śṛṅgākhya-sattvasya apadeśaḥ saṁjñā). Śrīla Jīva Gosvāmī confirms this in his Sandarbha: ruru-śabdasya svayaṁ muninaiva ṭīkā-vidhānāl lokeṣv aprasiddha evāyaṁ jantu-viśeṣaḥ. Thus although rurus are not seen in this world, their existence is confirmed in the śāstras.