Pfizer's earnings per share will only be reduced by the 20% minority interest making up the IPO. However, in return, the company is receiving $2.25B which they intend to use to buy back stock. Presumably, the increased earnings due to fewer shares outstanding will make up for the 20% of Animal Health earnings being lost.
Basically, you are not losing any earnings per share by dint of the ZTS IPO. If you think that the stock is overvalued here, then fine - sell. But ZTS should have nothing at all to do with that decision as overall it is not detrimental to earnings per share.
Sentiment: Buy