(mining) A mass of earthy matter, or rock of the same character as the wall rock, occurring in the course of a vein, as of coal or ore; hence, to take horse (said of a vein) is to divide into branches for a distance.
(US) An informal variant of basketball in which players match shots made by their opponent(s), each miss adding a letter to the word "horse", with 5 misses spelling the whole word and eliminating a player, until only the winner is left. Also HORSE, H-O-R-S-E or H.O.R.S.E. (see H-O-R-S-E on Wikipedia.Wikipedia ).