Unknown user in terminal

• What you see at the prompt at the terminal is a name assigned by your DHCP Server appended by your MAC Address

• The name of your Macintosh has been changed accidentally, or some program / app has changed it explicitly (or may NOT have a name in your case).

• Go to System Preferences and click on Sharing. Change the Computer Name. I am attaching the screenshot of the same.

• Once you do that Reboot you Mac. A standard name may look like "Steve Jobs' MacBook." Just personalize this with your own name. And at the prompt it would appear like Steve-Jobs-MacBook-Pro:~ steve$

A really simple way to sort this out is to simply go to System Preferences > Network > Advanced > TCP/IP, and simply make up any old name for the DHCP client ID (below the 'Renew DHCP Lease' button).