Código PHP:
$sql = mysql_query("SELECT n_caballo, nombre_caballo FROM race WHERE '$fecha_carrera' and hipodromo = '$hipodromo'and carrera = '$carrera' ORDER BY n_caballo ASC");
echo "<select name='n_caballo' id='n_caballo'>";
while ($registro1 = mysql_fetch_array($sql)) {
$n_caballo = $registro1["n_caballo"];
echo " <option value='$registro1[n_caballo];'>$registro1[n_caballo]</option>";
}
echo "</select>";