Katyayana (3rd or 4th century BCE) is also called Vararuci, the varttika kara (author of the varttikas). He is one of the three authoritative sages who composed commentaries on Sanskrit grammar. The other two sages are Panini, the sutra kara (author) of Ashtadhyayi Sutra, and Patanjali, author of Mahabhashya. Panini scientifically described the Sanskrit language, both Vedic and classical, in the form of aphorisms; Katyayana made scholarly observations in the form of criticism; and Patanjali provided judicious explanatory commentary, taking sutras and varttikas into account.
Varttikas of Katyayana are meant to correct modify or supplement the rules of Panini wherever necessary. Of the nearly 4000 sutras about 1500 were covered; doubts and objections were raised and in most cases, solved by addition, deletion, substitution, or correction.
Katyayana belongs to South India; Patanjali distinctly calls him a dakshinatya (southerner). Although according to Kathasaritasagara, Katyayana was a contemporary of Panini, the number of changes in the grammar observed by Katyayana and his criticism of them show that he must have come later than Panini, by about 100 years if not more, to allow time for so many changes in the development of later grammatical commentaries.
One Vararuchi is the author of Prakrta Prakasa, a grammar of the Prakrit language, consisting of aphorisms divided into twelve chapters that deal with general characteristics of Prakrit as well as special features of its three varieties. The last four chapters are spurious, according to some scholars. This Vararuchi is said to have flourished during the 3rd and 5th century CE. He is identified with Katyayana, the author of Sarvanukramani of Rig Veda, Katyayana Srautasutra of Shukla Yajur Veda and Katyayangrhya Sutra. He is also credited with the earliest Pali grammar and one Prakrita Manjari. The identification of Katyayana with Vararuci is believed to be based on the authority of Brihat Katha Manjari of Kshemendra, Kathasaritasagara of Somadeva, and Haracaritacintamani of Jayadratha.
Source - Encyclopedia of Hinduism Volume VI page 7 - 8 IHRF - Rupa 2011