True North Communications, Chicago, said it has acquired majority ownership of Arellano Publicidad in Mexico City. The agency, twelfth largest in Mexico, had billings of $34 million in 1994. Ignacio Arellano, who founded the company in 1970, will continue as director general. Arellano Publicidad will retain its current name and operate as the second independent agency of True North in Mexico. It also owns FCB Mexico, which had billings of $28 million in 1994.