The Collaborative International Dictionary of English v.0.48:

Horsemint \Horse"mint`\, n. (Bot.)
   (a) A coarse American plant of the Mint family (Monarda
       punctata).
   (b) In England, the wild mint (Mentha sylvestris).
       [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Mint \Mint\ (m[i^]nt), n. [AS. minte, fr. L. mentha, Gr. mi`nqa,
   mi`nqh.] (Bot.)
   The name of several aromatic labiate plants, mostly of the
   genus Mentha, yielding odoriferous essential oils by
   distillation. See Mentha.
   [1913 Webster]

   Note:

   Corn mint is Mentha arvensis.

   Horsemint is Mentha sylvestris, and in the United States
      Monarda punctata, which differs from the true mints in
      several respects.

   Mountain mint is any species of the related genus
      Pycnanthemum, common in North America.

   Peppermint is Mentha piperita.

   Spearmint is Mentha viridis.

   Water mint is Mentha aquatica.
      [1913 Webster]

   Mint camphor. (Chem.) See Menthol.

   Mint julep. See Julep.

   Mint sauce, a sauce flavored with spearmint, for meats.
      [1913 Webster]

WordNet (r) 3.0 (2006):

horsemint
    n 1: an annual horsemint of central and western United States
         and northern Mexico [syn: lemon mint, horsemint,
         Monarda citriodora]
    2: tall erect perennial or annual having lanceolate leaves and
       heads of purple-spotted creamy flowers; many subspecies grown
       from eastern to southwestern United States and in Mexico
       [syn: horsemint, Monarda punctata]
    3: a coarse Old World wild water mint having long leaves and
       spikelike clusters of flowers; naturalized in the eastern
       United States [syn: horsemint, Mentha longifolia]