(Reuters) - Pfizer Inc's (PFE.N) animal health unit Zoetis Inc filed for a $100 million initial public offering as the largest U.S. drugmaker narrows the focus on its core prescription drug business.

Pfizer, which bought U.S. rival Wyeth for $67 billion in late 2009 to bolster its array of medicines, has developed few big-selling drugs of its own since impotence-treatment Viagra was introduced in 1998.

The company has been cutting costs across its business, including paring research spending, as its blockbuster drugs like cholesterol fighter Lipitor face increased competition from cheaper generics.

To bring the focus back on its core business, the company recently agreed to sell its baby formula business to Nestle SA (NESN.VX) for $11.85 billion and sold its Capsugel unit, the world's largest maker of hard capsules, to private equity firm KKR & Co (KKR.N) for nearly $2.38 billion last year.

Pfizer said in June that it planned to separate its animal health unit, which sells medicines, vaccines and other products for livestock and pets, into a standalone company. The business, with revenue of $4.2 billion last year, has more than 9,000 employees.

Zoetis competes with the animal health businesses of Merck & Co (MRK.N), Eli Lilly and Co (LLY.N), Sanofi SA (SASY.PA), and Novartis AG (NOVN.VX).

The company sells products across five major categories, with livestock products representing about 66 percent of its revenue.

Zoetis makes Palladia, the first drug to be approved by the FDA for treating cancer in dogs, and has developed the first swine vaccine for pandemic H1N1 Influenza Virus in the United States.

IPO TERMS

In a filing with the U.S. Securities and Exchange Commission, Zoetis said Pfizer will exchange Class A shares of the unit for its debt held by certain of its debtholders, who in turn will sell it through the offering.

Pfizer and Zoetis will not receive any proceeds from the offering of the Class A shares, but the parent company will own 100 percent of Zoetis' outstanding Class B stock.

Both Class A and Class B shareholders are entitled to one vote per share on all matters except the election of directors where Class B shareholders will get 10 voting rights.

Pfizer expects to divest up to 20 percent of its stake in the IPO.

JPMorgan, BofA Merrill Lynch and Morgan Stanley will underwrite the offering.

Pfizer Chief Executive Ian Read had said the company was targeting the completion of the IPO in the first half of 2013, giving it more options to fully separate the business.

The filing did not reveal how many shares the company planned to sell or their expected price. The company did not disclose the ticker symbol and exchange it planned to list its shares on.

The amount of money a company says it plans to raise in its first IPO filings is used to calculate registration fees. The final size of the IPO could be different.

(Reporting by Sharanya Hrishikesh and Jochelle Mendonca in Bangalore; Editing by Don Sebastian)