Platypus

The platypus, or duck-billed platypus, was an Australian animal that survived into the 20th century. Its most notable traits were its duck-like beak and the fact that, despite being a mammal, it laid eggs as part of its reproductive cycle. (PROSE: The Doctor Who Dinosaur Book) This peculiarity was known to Zoe Heriot, who used them as a point of reference when trying to understand the biology of the Ulk-Ra. (PROSE: War Crimes)

The Poldrantiax were said to resemble a cross between a platypus and a squid. (WC: U.N.I.T. On Call)