"Charles feared that the big and scary dog would jump over the short metal fence." A comma should be added between:

"Charles feared that the big and scary dog would jump over the short, metal fence."

A comma should be placed between the adjectives that describe the noun fence. The reason is that they are not joined by a coordinating conjunction as in the description about the dog: big and scary dog. As a result, since they appear in the form of list without any linking words, they must be separated by a comma.