It's a spoken, non-standard version, mostly encountered in the southern regions of Russia. It's used to be considered as inappropriate for the formal style, however it may be found in fiction books and in the spoken language: 'ихний, ихняя, ихнее, ихние' - 'their'; and even 'евонный' for 'his' and 'ейный' for 'her(s)'.