Duck-billed platypus quick facts:
- About the size of a housecat (2-5 lbs)
- Swimming rate should be about 10 ft (real world movement ranges 1.5 - 2 ft per second).
- Land movement should be about 5 ft, with sprints up to 20 ft.
- The duckbill is fleshy and very sensitive; the platypus uses it to find food in muck, and to help guide itself through murky waters.
- Digging rate should be about 1 ft, which is really impressive in the real world, but in D&D might be altered to 5 ft. It's a magical platypus

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"The venom is potent enough to kill dogs, but in a human it causes severe pain, which is not alleviated by analgesics. The pain can last for at least 36 hours and the limbs may swell up like balloons. Following this, a rash can last for several months. A potential gamma-globulin antibody has been found for the venom, which could lead to an antivenene to treat a person who has been spurred. " -
http://www.platypus.org.uk/det-descrip.html
D&D's poisons are not realistic, so the above can probably be ignored in favor of a weak DEX poison.