Pfizer Inc. is taking its animal-health unit public in the biggest U.S. initial offering since Facebook Inc., a bet that growing affluence means more spending on caring for livestock and pets.

Pfizer’s Zoetis Inc. is seeking to sell 86.1 million shares today for $22 to $25 each, giving the unit a market value of about $11.8 billion at the midpoint of the range, regulatory filings show. The IPO, which may raise as much as $2.2 billion, would make Zoetis the largest public company of its kind — and one of the only focused solely on medicines for animals, competing with businesses owned by such drugmakers as Sanofi and Merck & Co.