Mr. Pinchy is only fished out of Highland Mixed Schools in Terrokar Forest. There are a total of 17 Highland mixed schools in the three Fly-to only lakes in Terrokar Forest. 3 in the lake above Stonebreaker point (lake Jo'rune), 4 that spawn in the lake just above and behind Allerian Stronghold (Lake Ere'Noru), and 10 in Blackwind Lake/Skettis area. Mr. Pinchy's drop rate appears to be about a 0.2% drop rate on successful casts (about 2 out of 1000 or 1 in 500 casts) so be prepared to spend alot of time on this.

You will need 420 fishing to Cast into the Highland Mixed schools, and about 480 to loot without many "Your fish got away!" messages.
You will no longer receive "Your fish got away!" messages if your fishing skill is 525 fishing or higher.

I had the games max limit of 527 Fishing the 3 times that I have caught him:
375 Base Fishing Skill
+40 Arcanite Fishing Pole (I have the High Test Eternium Fishing Line on it so 35+5=40)
+5 Lucky Fishing Hat
+5 Nat Pagle's Extreme Anglin' Boots
+2 Fishing Skill enchant to gloves
+100 Aquadynamic Fish Attractor

Upon Catching you will get one of the following results (listed in what seems to be the most common results first)

1 - Mr. Pinchy's Gift, contains 4-5 Super Healing, and 4-5 Super Mana potions, this seems to be the most common result of his wishes.
2 - Mr. Pinchy's Blessing, 2-hour 1200hp buff that persists through death!
3 - Furious Mr. Pinchy, Lvl 70 non elite that attacks you and your party, is not lootable.
4 - Benevolent Mr. Pinchy, Lvl 70 Guardian that assists you for 10 minutes
5 - Magical Crawdad Box, summons a Magical Crawdad non-combat pet, very rare
considering such a low drop rate on Mr. Pinchy to begin with, and then only 3 chances to get this box. Otherwise, time to try to get Mr. Pinchy again for another 3 chances!

Seravais <Mixed United Tribe> - Lvl 70 Undead Rogue - Medivh - Horde
2 time Master Angler, Bloodsail Admiral, and owner of over 40 non-combat pets