From A Wiki of Ice and Fire
The manticore is a legendary creature with a human head, a lion's body and the tail of a scorpion. This legendary being appears to have been relatively common in the Valyrian statuary, and is represented as one of the gargoyles that adorn the fortress of Dragonstone. The manticore is the charge on the arms of House Lorch. The Manticore Isles are presumably named after the creature.
Manticores are known to have very unsettling human-like faces and can fold up into the shape of a scarab  They are rumored to live on the islands of the Jade Sea. 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 and maesters.
A Game of Thrones
A Clash of Kings
A Feast of Crows
Qyburn claims that the poison Oberyn Martell coated his spear with and used against Gregor Clegane was manticore venom, but thickened somehow possibly with sorcery. Instead of killing instantly when it reaches the heart, the modified venom leaves Clegane to die slowly and in extreme agony.