I still can't figure out what this means.

Are they deleted accounts?

It is e621's name of Anonymous.

I have a related question, it seems that the "Guitar" part of the name is italicized, why is that?

Euphemism?

I also thought it was a user XD

Fox2K9 said:
I also thought it was a user XD

I didn't know until I tried clicking.

I thought it was some loser talking to himself.

Double post.. really.

"The dog in your guitar"

Gentlemen, I refer you to this cartoon:

http://www.fat-pie.com/ptikobj.htm

or this

http://e621.net/post/show/64189/

if your paranoid about visiting unknown sites

And that exact same link is in the source. -_-