Megalodon

The Megalodon's size is determined by the size of its tooth. The Megalodon's weight is determined by it's length according to several formulas.

They lived in the Mioune and Plioune eras about 1.6 million years ago. The megalodon can weigh 47 to 103 metric tons ;sharks eat about two percent in body weight. Scientist estimate that the Megalodon could be up to 50 to 100 feet in length.

http://www.greatsouth.net/fossil_replicas/FR182_Megalodon_Shark_Tooth.htm
http://www.enchantedlearning.com/subjects/sharks/glossary/Megalodon.shtml
http://en.wikipedia.org/wiki/Megalodon#Size_estimation

Box jelly fish

The Box Jelly fish is deadly by it's poison. When it's needle like tentacle touches your skin a sensor tells the tentacle to let toxic chemicals into the skin. These chemicals could travel into the heart and kill a human in a matter of minutes. About twenty to forty humans die a year. Most of the big box jelly fish would kill you in almost 2 minutes. There are 50 species of jelly fish of box jelly fish in the world.

http://wwlifeslittlemysteries.com/how-deadly-is-the-box-jellyfish--0734/