Okay, so it’s a touch of both.

For starters, his name is Tad Strange. He’s going to be voiced by Cecil Baldwin, and he IS a canon character, but that’s all we know for certain. 

Then there’s this from Bill Cipher’s AMA (which, if you haven’t followed, was basically a mass public interview with Bill. Actual Bill. Alex Hirsch was in charge of it.)

So, things we know for certain:

• Tad Strange is a Gravity Falls character voiced by Cecil Baldwin
  
• Tad and Bill know each other.

Assumptions being made

• Tad may LITERALLY be a square, in much the fashion that Bill is a triangle. Bill sounds kind of literal about it (real square), though he may have just meant Tad’s lame.
  
• “Tad Strange” sounds like a pseudonym much the way “Bill Cipher” is. Bill confirmed that his name is just a pseudonym because his real demon name cannot be heard or comprehended by human ears.
  
• Bill sounds familiar with Tad. More like a peer he finds annoying rather than a sniveling lower life-form (human).

All in all, this adds up to an extremely loose (but interesting) theory that Tad is a square demon. A lot of people draw him purple as a reference to the color scheme of Welcome to Nightvale, which Cecil Baldwin is widely known for. I’ve also seen designs of him that resemble a floppy disk, because of the cryptic message from the mysteryofgravityfalls website “don’t copy that floppy.” There’s one depiction of him as the all-knowing being on the other side of the mailbox from the gravity falls short, and another based on the illuminati square and compass. Scroll through here.

Additionally, there’s a theory that he may be linked to Gompers. People have been suspicious of Gompers’ slit-eyes for a long time, since they’ve got the same shape as Bill’s (but sideways). Not to mention the long lore behind goats and devils/demons.

Then this, also from the Bill AMA

Whether or not Tad’s involved, there IS something up with Gompers. Either Bill did something to Gompers, or was around to witness something done to him, and likes the change. People like the idea that Tad is Gompers, and has been turned into a goat as punishment.

In summary, this stuff is like 95% wild mass guesses and headcanons right now. Tad Strange might be a used car salesman from Denver for all we know, but he is a character. The fandom right now is just having fun assuming that he’s a purple square demon on bad terms with Bill, and has since been turned into a goat.