"Dougal - you settle down now, please."
-- Newt Scamander (WFT)
Dougal was a Demiguise who lived in Newt Scamander’s case (WFT).
Dougal was one of the creatures that escaped from Newt Scamander's case in New York. Because Demiguises can turn invisible, Dougal was especially elusive. He babysat the escaped occamy in Macy's. Gnarlack tipped Newt off about where Dougal might be (WFT).
Other canon notes and references
Dougal was restless while entering New York Harbor aboard a ship and again when Newt was pursuing the NIffler at the bank, and Newt had to ask him to calm down (WFT).
Dougal seems to be fond of sweet food. He would steal sweets, but he also tried to share them with the Occamy in Macy's (WFT).
Dougal seemed to like Jacob Kowalski (WFT).