Drop: Monstrous Spineclaw 
Zone: Timeless Isle
Approximate Drop Rate: 1 in 58
100% Hit Chance
Deals 275 Beast damage and causes the target to Bleed for 98 Beast damage per round for 5 rounds.
Wildly flails at the enemy 1-3 times, dealing 197 Beast damage per hit.
Deals 98 Aquatic damage each time you take direct damage. Effectiveness is tripled for 1 round if this ability is used while already active.
Lasts 5 rounds.
Restores 591 health to the user.
Causes a whirlpool to form under the enemy pet.
In two rounds, the opponent's pet will take 591 Aquatic damage and be rooted for 2 rounds.
50% Hit Chance
Deals 788 Aquatic damage.
Always hits if the target is Bleeding .
Harmful damage over time effects are reduced by 50% on Aquatic pets.
50% from Flying abilities
33% from Undead abilities
1. Any Ancient Spineclaw killed, when it respawns has a chance to spawn as the Monstrous Spineclaw.
2. ONLY the Monstrous Spineclaw can drop the Spineclaw Crab pet.
3. The Spineclaw Crab pet has a drop rate from the Monstrous Spineclaw of 1.8% per Wowhead (6/19/2014).
4. The "chance" that a Monstrous Spineclaw will spawn in place of an Ancient Spineclaw is the same as the other rare timers, every 24 - 64 minutes (same as Houlon.
Wow. I feel it can only be due to the limited number of people actually having this pet that it isn't higher rated.
By now I've collected at least 13 out of the 20 most powerful pets according to this site, and my P/P Spineclaw Crab still tops them all. Rip (attack + powerful dot), Healing Wave (tends to go for 550+ every 3 rounds) and Blood in the Water (hits for 750+ every three rounds) just annihilate whatever is in the way. Plus, it has amazing aesthetics as well.
one could only hope the drop rate was 1 in 58................................................. I'm about 150-160 kills deep right now & no sign of a spineclaw, guess I'll down 150 more!
Farming the Spineclaw Crab can be quite frustrating.
Best farming spot: The shore just south of the Jademist Dancers and north of the Horde camp. Seven Ancient Spineclaws spawn around there. Helps to use Scuttler's Shell.
Monstrous Spineclaw spawn rate: Any Ancient Spineclaw can spawn as a Monstrous Spineclaw instead. I'd say about a 1/100 spawn rate (No data to back that up).
Spineclaw Crab drop rate: Wowhead says 1.7%. Other Timeless Isle rares seem to drop pets at 3%.
I absolutely love this crab (P/P) and the Emperor Crab (P/P), both have 357 attack power and murder critters & elementals. Rip+SC+BitW=Elemental Destroyer. Tripple Snap=Critter Destroyer (altough rip is also good vs critter).
The one that I managed to acquire was indeed Blue, cobalt blue in fact..looks just like the Ancient spineclaws you kill to spawn the Monstrous. He is quite impressive :)
So when I am killing spineclaw crabs, does it have a chance to spawn where I am killing them or anywhere that spineclaw crabs can spawn on the island?
is this guy blue or green? ive seen picture of him being green, i like the blue better hoping for that IF i ever see him lol
Everytime you kill an Ancient Spineclaw, there's a chance that a Monstrous Spineclaw will spawn on the island. It can spawn in any of the Ancient Spineclaw spawn locations. Same mechanic works for the rares: Emerald Gander and Ironfur Steelhorn (they don't drop pets though) and Imperial Python (does have a pet)
Note that this drops from a rare spawn crab, not from the regular elite crabs found all along the shore.