A polyfill adds missing functionality using monkey patching. For example, this can get you Array.prototype methods in Javascript such as .map and .reduce. This can be dangerous as it affects all code running in the environment, as other pieces of code may rely on this unbeknownst to the developer.
Essentially a type of [[shim]] for the browser.
A ponyfill doesn't patch anything, but provides functionality as a standalone module you can use. It avoids altering the 'default' API, while allowing you to use it eplicitly.