https://www.cnet.com/roadshow/news/ford-patent-application-awd-hybrid-v8/

Of course, a V8 isn't mandatory -- it will apparently work with any longitudinally mounted internal combustion engine -- but the fact that the patent application sketches involve a V8 means we might be looking at the hybrid system for the upcoming Mustang hybrid, and perhaps the F-150 hybrid, too.

AutoGuide notes that Ford once promised "V8 power and even more low-end torque" in the Mustang Hybrid, but it's possible that first part of the quote might refer to a literal V8 and not just a pony-for-pony recreation of a V8's output with a smaller motor coupled to the hybrid system. This could be backed up by Ford's own ad, which may have shown a V8 engine as it teased the Mustang hybrid for the first time, but all Ford has said thus far is to stay tuned for future updates.