Top-notch companies don't often come cheap. Zoetis (ZTS +0.70%) is one of the world's leading animal healthcare companies. The stock has traded at an average price-to-earnings ratio of nearly 39 over the past decade, a lofty premium to the broader market.Today, Zoetis stock trades at just 19 times earnings, its lowest valuation since the stock began trading. Stock prices routinely fluctuate, but such a stark departure from long-term trends warrants a closer look at the business. Here's why Zoetis has slippe ...