Like the Elephanilla (see answer), this is another pinata that takes time to reside. As with any pinata that needs multiple visits, it's important to keep in mind that you probably won't get the exact same Chippopotamus, the next time a wild one visits your garden.

Always make sure to maintain well more than 14 watercress or 14 water lilies or 14 bullrush in the garden, and replant the ones it eats. If its Visit requirements are no longer met, it will leave the garden early, drawing out the number of visits needed.

If you've got a visitor that only needs to meet one or two more requirements, try not to distract it with other items that it no longer is required to eat. One tip is to whack the plants that a particular visitor needs to eat, and move the flower heads near where he visits/eats, to help speed things along. This way, it will hopefully eat more of what it needs to eat, instead of randomly devouring whatever it's fond of.

Ultimately, patience is the key, while you pay attention to its needs, and keep your garden well-planted. (Either that, or you could pay someone off to ship you one.)

According to an in game rumor if you feed a Rashberry 10 pumpkins it will turn into a chippopotamus   View thread