Poison Pokeys are greenish-blue poisonous Pokeys encountered at Riverside Station and the Pit of 100 Trials. They are tougher than regular Pokeys and have more health. Obviously by the name, they can inflict poison to Mario, gradually decreasing his health.
Note
Poison Pokeys are NOT the same as Pokey Mummies. Poison Pokeys have gained their poison ability by mutation or an inherited trait. Pokey Mummies are dead Pokeys that have risen from the grave and somehow gained the ability to become poisonous. They are weak to Mario's hammer. Poison Pokey also has twice amount of strength than Pokey Mummy.