Just as Jahtees are known to be 6, Vedas are known to be 4, Puraans are 18; same way Birs are known to be 52. Mahankosh has provided a long list of these 52 Birs. The only one that is recognizable is Hanuman. Others are not famous.
Birs, I believe are spirits that many Tantriks (ones who do Black magic) try to control. It is said that Baba Banda Singh Bahadur, prior to becoming a Sikh controlled many Birs. When Guru Sahib went to his Dera and sat on his throne, he sent Birs to dismount Guru Sahib but none of the Birs were able to move the throne on which Guru Sahib was seated. Then in the end, Madho Das Bairagi (Baba jee's previous name) himself came to Guru Sahib and after some questions, submitted himself to Guru Sahib.
Kulbir Singh
ਸਤਿਗੁਰਬਚਨਕਮਾਵਣੇਸਚਾਏਹੁਵੀਚਾਰੁ॥