Scorpion
Scorpions are known as small reptiles that have the most poisonous in the world. Scorpions enter the class of animals arachnida and still siblings with other stars such as spiders to mites. This animal is quite feared, because with its small shape allows it to easily enter into small cracks so we do not see it and then stung by can who was in the tail.
@khairaaluna please note that scorpions are not reptiles nor arachnids. There are two distinct families: the Buthidae and Scorpionidae, the second of which being mostly harmless.