Drop: Monstrous Spineclaw 
Zone: Timeless Isle
Approximate Drop Rate: unknown
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
I was killing frogs on Timeless and waiting for Bufo to spawn when Monstrous Spineclaw spawned to the bottom of the ocean next to the frog place. It was maybe the second time I've ever seen that and was really surprised that the pet dropped! Happy to have it although feel kinda bad that my sister who's more active pet collector than I am didn't got this before I did.
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.