Final answer:
The comma should be added between 'feared' and 'that' to separate the introductory phrase from the rest of the sentence.
Step-by-step explanation:
The comma should be added between feared and that.
The sentence should read: Charles feared, that the big and scary dog would jump over the short metal fence.
The comma is necessary to separate the introductory phrase 'Charles feared' from the rest of the sentence.