The Collaborative International Dictionary of English v.0.48:

Foal \Foal\ (f[=o]l), n. [OE. fole, AS. fola; akin to OHG. folo,
   G. fohlen, Goth. fula, Icel. foli, Sw. f[*a]le, Gr. pw^los,
   L. pullus a young animal. Cf. Filly, Poultry, Pullet.]
   (Zo["o].)
   The young of any animal of the Horse family (Equid[ae]); a
   colt; a filly.
   [1913 Webster]

   Foal teeth (Zool.), the first set of teeth of a horse.

   In foal, With foal, being with young; pregnant; -- said
      of a mare or she ass.
      [1913 Webster]