A nightflyer is likely a type of owl on Equellus.
When Red-Eye asks Vella and Puppis about the "strange sound" heard by Lacerta in town, Puppis claimed that he heard a "nightflyer hoot down by the woods" (The Lost Warrior).
Original Series definition: an owl-like creature on Equellus that hoots; Red-Eye asks Vella and Puppis about the "strange sound" heard by Lacerta in town, to which Puppis offers that he heard a "nightflyer hoot down by the woods" (TOS: "The Lost Warrior"). (TOS: "The Lost Warrior")