A long long long long long time ago ... 1989 to be exact ... There was a PC game that came out called "Wing Commander". This was a space combat flight simulator type game where you're basically a pilot and flying space fighter jets against alien cats ... so yeah, its like Star Wars but you fight giant talking cats.
Anyways, when you first load the game up it asks you to enter your last name and call sign. So, like probably every other 8 year old kid at the time who had watched "Top Gun" 50,000 times by that point in life.... My first instinct was to put "Maverick" in for my call sign.
However, me being the bastion of originality that I am ... had the thought ... "Wait a minute, every other kid in the country is going to be 'Maverick' on here!" ... And I didn't want to do that, but I still wanted something that referenced "Top Gun" .... So, I remembered the scene where Maverick and Goose were getting their asses chewed by their CO for helping Cougar land his plane, and they were being sent to Mirimar.
So, a line in that scene was "If you screw up even this much ... You'll be flying a plane full of rubber dog **** out of Hong Kong!" .... But obviously, I wasn't going to use "Dog****" as my callsign, so somehow my 8 year old brain came up with "Dogmeat" ....
So I've gone by "Dogmeat" on everything from oldschool BBSs, to AOL, to IRC, to web boards, everything ..... since 1988 or 1989 .... LoL.