Apple introduced a new fixed width font in Xcode 8 — San Francisco Mono — but they’ve not made it available for system-wide usage.

If you want to make the fonts available for system-wide usage, you need to extract them from the Xcode 8 beta and add them to Font Book.

Assuming Xcode is installed in /Applications, navigate to the below folder using Finder:

/Applications/Xcode-beta.app/Contents/SharedFrameworks/DVTKit.framework/Versions/A/Resources/Fonts

Then select the fonts in that folder and drag them to Font Book.