The best way for virtual pet site owners to make money from their virtual pet site is by having your site Generate revenue from your website. Google AdSense. which is a advertising company (by google) that pays publishers via clicks, ECPM (the higher eCPM your pet site has the higher your earnings will be), however; you need to know one important fact. Adsense stinks if your virtual pet site doesn’t have enough content opened to the search engines or guests - as adsense’ bots need to crawl your sites pages to display relevant ads to your pet site. Neopets and Marapets both use Adsense as one of their advertising company, however; if you want to diverse your advertising revenue, then be sure to look for other solutions.You may want to look into CPM (cost per thousand) programs such as ValueClick, TribalFusion, CasaleMedia, etc… These programs all pay based on your games page views. Each advertisement you place will have a rate. Lets say its .25/CPM. That is 25 cents you earn per every 1,000 impressions. This is often a good option for pet games as they generate tons of page views but do not require users to click on the ads. Most games will have the option to “donate†for special features and items. Please realize that these are not really donations, and you do have to pay taxes on them. For “donations†to work well you need a good system in place. You should use paypal IPN to instantly communicate between your server and paypal so you can provide real time transactions. When designing packages for users to buy, try to make them appealing but don’t make it unfair for players who cannot afford to donate. http://www.virtualpetsblog.com/2007/10/30/making-money-from-your-virtual-pet-site/
Neopets is a grand example (quite old) but they're monetizing in MANY ways and generating a huge amount of income.