Silver, e.g. (5)
(Other definitions for horse that I've seen before include "Steed" , "Suffolk punch" , "This common sense is of equine origin" , "Nag, hack" , "Its mouth a reliable source of info" .)
(Other definitions for metal that I've seen before include "Copper, perhaps" , "Tin, for example" , "lead as one" , "Copper or silver, say" , "barium?" .)