This was a new one on me, while the usu on-line dictionaries weren't of much help

In other venues I get a variety of responses, some of them conflicting

So far, here's my best hit:

HipHopCanada.com :: View topic - Today's Urban Word of the Day ...Urban Word of the Day: porch dog A person who frequently attacks others in ... Origin: The phrase "porch dog" is used to refer to dogs that sit on front ...
www.hiphopcanada.com/_site/community/phpBB2/ viewtopic.php?p=331095&sid=193dcd9dc3f2b079d2bf7170f552ea35

quote: porch dog A person who frequently attacks others

Sounds more like a junkyard dog. Never heard 'porch dog' before.

I think of a junkyard dog as a trained killer, whereas a porch dog is just territorial.

There were 61 of about 137 sites for "porch dog/origin." Someone on another forum quoted the Urban Dictionary's definition:

A person who frequently attacks others in speech or writing, but who
poses no intellectual threat whatsoever. The motivation of this type of
person can usually be accurately construed as a desire to be obnoxious
and offensive.

Origin: The phrase "porch dog" is used to refer to dogs that sit on
front porches and bark (vigorously and fruitlessly) at passersby, but who
pose no physical threat.

Yeah, that guy has a scathing response to just about everyone who posts
in this forum. He's a real porch dog.

Apparently it is another word for a troll.

[I think I won't go into what they said about porchmonkey. ]