what's the meaning of this setence :"It is the nest of the h

what's the meaning of this setence :"It is the nest of the horse that I put my nose into there."This sentence is drawed from a famous fiction written by Agatha Christie."……‘I have indeed been foolish to take the matter so seriously,’ said Poirot.‘It is the nest of the horse that I put my nose into there.’ ‘You’re mixing up mares and wasps,’ said Japp.……"You can translate it into chinese,thanks for you generous help.