* They are both animals: This is the most basic commonality.
* They both have a sense of smell: While a rhinoceros' sense of smell is incredibly strong, ducks also use their smell to find food.
* They both live on Earth: This is obvious, but it highlights the shared environment they inhabit.
The real trick is that there isn't a particularly interesting or obvious connection between them.
Perhaps you're looking for a more creative answer? Do you have a specific thought in mind?