There are four Kaatyaayan known. We are talking about the fourth Kaatyaayan.
(1) Kaatyaayan, The Economist in Dasharath's Court
There was one Kaatyaayan, an economist, in the court of King Dasharath,
Raam's father, in Ayodhyaa. He had a sister named Kaatyaayanee whom he
married to Yaagyavalkya. She was a simple girl.
(2) Kaatyaayan, The Prime Minister of Magdh
There was one Kaatyaayan in the history of India too during Chaanakya and
Chandragupt period. There was a young king of Malaya, named Chandraketu.
Chandragupt Maurya defeated Nand, the King of Magadh with the help of
Chaanakya and Kaatyaayan (the former Mahaa Mantree - Prime Minister of Magdh
King Dhanaanand) and ascended the throne of Magadh.
(3) Kaatyaayan, A Grammarian
He is said to be living during 240-160 BC. Kaatyaayan was an ancient grammarian.
He wrote two books - "Vaartik" - an elaboration on Paanini grammar.
Along with Mahaabhaashya of Patanjali this text has become a core part of the
grammar cannon. This was one of six Vedaang and constituted compulsory education
for the students for the following 12 centuries. He composed "Sulabhaa
Sootra" also - a series of 9 texts on the geometry of altar constructions,
dealing with rectangles, right-side triangles, rhombuses etc.
(4) Kaatyaayan Rishi
Kaatyaayan was born in the family of Katya Rishi that is how he came to
be known as Kaatyaayan. It is believed that Maharshi
Vishwaamitra had a son named
Kat from among his many sons. Kat had a son named Katya and Katya's
descendents are called Kaatyaayan. Thus he was in the lineage of
Once Kaatyaayan Rishi did a severe penance to please Devee and asked her
boon that he wanted her to be born as his daughter. She granted the boon
and then she was born in his house. Rishi named her Kaatyaayanee. She killed
the demon Mahishaasur
Sookt of Rig Ved have been created by various Rishi. They were first of all
described by Kaatyaayan Rishi in Rig Ved Sarvaanu-kramani.