Male Playpuses have small spurs on the back of their front heels that secrete a mild venom. Even though it isn't fatal to humans, it has been known to cause much pain to those that are unlucky.
platypus!!
Arlen Specter
No, an adder is not a mammal. Adders are venomous snakes found in Europe and Asia.
No. They are very territorial and VERY dangerous but not poisonous.
A snake
The adder is the only poisonous British snake, and is a reptile.
A slow loris is the only poisonous mammal in the world.
No, because the platypus is not a marsupial. A platypus is a monotreme (egg-laying mammal). It is not the only venomous mammal. "Venomous" is the correct term, rather than "poisonous", as things which are poisonous must be ingested, i.e. eaten or drunk. Other venomous marsupials include the Cuban solenodon, Eurasian Water Shrew, Northern Short-tailed Shrew and Southern Short-tailed Shrew, just to name a few.
There is no male mammal that has a poisonous spike that sticks out of its hind leg. However, adult males platypuses have a hollow spur on each of their hind legs, through which they can deliver a venom strong enough to kill a small dog, and to cause almost paralysing agony to an adult human. The spur is connected to a venom gland in the platypus's thigh.
Sharks aren't mammals. They are fish, and no. Lots of other fish are more poisonous. Sharks have the best attack though.
The only mammal that can sting is the Platypus. Male platypuses have spurs on their back ankles with from which they can inject poisonous DLPs (defensin-like proteins). Female platypuses and echidnas also have these back ankle spurs, but only male platypuses can use them.
Primate