Manticores

"The cause appears to be manticore venom...terrible poison..."

- Grand Maester Pycelle

Manticores are highly aggressive and extremely poisonous scorpion-like insects from the continent of Essos.

Biology
Manticores have an overall body-plan similar to scorpions, but can grow almost as large as a lobster. They possess six legs and larger stinger-tail, which curves up over the body to hang over the head. They have a series of threatening-looking spines on each body segment, running up the sides of the body and tail. They are mostly green in color.

Their most recognizable features are the designs on the plates of their bodies, particularly the marking on the final plate of their tails which resembles a human face. Because of this design they are also sometimes referred to as the "Death's Head Manticore".

Manticore venom is extremely lethal, and even a single sting is usually more than enough to kill a person. Extracted manticore venom is one of the most potent poisons which are used by the world's assassins, sometimes by coating bladed weapons with it, so that even a single scratch will kill. Extracted manticore venom is known to be produced in the city of Mantarys (east of the Free Cities and west of Slaver's Bay).

Season 3
An envoy of the Warlocks of Qarth attempts to assassinate Daenerys Targaryen using a manticore hidden inside a hollow wooden sphere. Before the creature can harm Daenerys with its lethal sting, Ser Barristan Selmy impales it with a dagger, killing it.

Season 4
Grand Maester Pycelle and Qyburn assess that Oberyn Martell coated his spear with manticore venom prior to his duel with Ser Gregor Clegane. While Gregor won the duel by crushing Oberyn's head in, the poisoned wounds he took left him in agony, and he is now comatose. The effects of the poison are horrific, making the flesh around Gregor's wounds putrefy, which produces an overpowering stench. Pycelle regretfully tells Queen Cersei Lannister that Ser Gregor is beyond any hope of healing, but Qyburn insists that he can use the knowledge he has gained from unethical human experimentation to save Gregor's life. Cersei gives him permission to proceed, though he warns that the process will "change" Gregor.

In the books
In the A Song of Ice and Fire novels, manticores are venomous insects from Essos, said to live on the jungle-filled islands of the Jade Sea, between Qarth and distant Asshai. They have a poisonous sting that is fatal to humans and kills the instant it reaches the heart unless a clotting agent is used to thicken it and delay its fatal effects. Manticore venom is a highly sought after poison by alchemists, Maesters, and assassins. Thus the insects have been imported into private collections across Essos and Westeros.

When treating Gregor Clegane, Qyburn theorizes that Oberyn used his extensive knowledge of poisons to add a clotting agent to the manticore venom he coated his spear with, thickening it in order to slow it's progress through the veins towards the heart, dragging out Gregor's agony. This also explains why Oberyn didn't finish off Gregor when he had the chance: he never intended to give him the mercy of a quick death.

Within the cultures of Westeros and Essos, the legendary manticore (after which the insect takes its name) is a chimeric creature with a human head, a lion's body, and a scorpion's tail. Dragonstone is adorned with gargoyles in the shape of manticores. A manticore is also the sigil of House Lorch.