SB 5.26.11
ye tv iha yathaivamuna vihimsita jantavah paratra yama-yatanam upagatam ta eva ruravo bhutva tatha tam eva vihimsanti tasmad rauravam ity ahu rurur iti sarpad ati-krura-sattvasyapadesah.
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
In this life, an envious person commits violent acts against many living entities. Therefore after his death, when he is taken to hell by Yamaraja, 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.
Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
According to Sridhara Svami, the ruru is also known as the bhara-srnga (ati-krurasya bhara-srngakhya-sattvasya apadesah samjña). Srila Jiva Gosvami confirms this in his Sandarbha: ruru-sabdasya svayam muninaiva tika-vidhanal lokesv aprasiddha evayam jantu-visesah. Thus although rurus are not seen in this world, their existence is confirmed in the sastras.