I've randomly fished off and on for a couple hours here and there since BC trying to catch Pinchy. No luck.
Several months ago I was fishing...approaching the 3 hour mark. My server is RPPVP but normally I can fish in peace. Not this day. So, my brave protector flies out to rescue me. He lingers beside me to ward off any would be attackers. Decides to cast his fishing line in to to the water. I just finished saying "I swear to God if you get my fish I'm going to be SO FURIOUS with you" when I look in party chat and see "Leo recieves (whatever it says when you loot Mr Pinchy)". Swearing furiously I mount up and fly off cursing the day people at Blizzard were born having fits about RNG. He laughs hysterically (we're in skype) mentioning this is the first time he's ever fished up there which only makes me more angry.
After reading something Quintessence wrote here the other day about a rarespawn perhaps dropping Mr Pinchy in WoD I decided to finally breakdown and try fishing again since competing for a rarespawn on my server for a maybe drop does not sound like my idea of a good time. (I have the pet already, btw, this just happens to be the last portion of the meta fishing achiev I need).
Fortunately, RNG was my friend that day and I got Mr.Pinchy after about half an hour. Clicked Mr.PInchy's box, got the pet and my achiev/title all at once.
Moral of the story:: It is completely random. You could fish for hours everyday for years and never see it or you could catch it on your first cast.
Good luck though.