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.

The IPO is well-timed — coming as stocks hit five-year highs — and represents another step by Pfizer chief executive officer Ian Read to slim down and focus on developing new prescription drugs. Both Novartis AG and Bayer AG explored a purchase of Zoetis last year, according to people familiar, reflecting the attractiveness of the animal-health industry.

“Globally, people are feeling wealthier, which tends to mean more demand for meat for human consumption,” said Erik Gordon, a professor at the University of Michigan’s Ross School of Business. “They’re also more likely to have pets and more likely to take better care of their pets.”

Pfizer, the world’s largest drug company with $61.2 billion in sales, is offering 17 percent of Zoetis in the IPO, the biggest in the U.S. since Facebook raised $16 billion last year.

Zoetis, based in Madison, New Jersey, traces its roots back to 1952 as a Pfizer unit and has made at least 10 acquisitions to become the largest animal-health business, with $4.3 billion sales in 2012 — about 20 percent of the $22 billion market.