Throg was a (presumably fictional) crewmate to Brian Johnson. Due to not having enough mental stability to travel in space, he started to think that the room blacked out when it hadn't. Eventually he went mad and thought his own foot was a fish, which he caught and ate. Brian deliberately told this story to unsettle Lucy Barker. (Full Metal Sophie) Given the reason Brian had for telling the story, to convince her she couldn't become an astronaut, Throg was likely a fictional character.