Green’s Dictionary of Slang

‘a spy quartered with an accused person with a view to obtaining incriminating evidence’ (OED).

1804		Edinburgh Rev. Jan. 442: His friends were denied access to him, and a mouton or jail-spy quartered in his chamber .
1902		A. Lang Hist. of Scotland II 239: A mouton or prison-spy, named Herle, had extracted much of the truth from Bailey .