This name is a synonym of Isodon megathyrsus (Diels) H.Hara.
The record derives from WCSP
(data supplied on 2012-03-23)
which reports it as a synonym (record 454017) with original publication details: Mem. Fac. Sci. Taihoku Imp. Univ. 2: 131 1929.
This name record (from WCSP) was not included in version 1 of The Plant List. It is possible the same name was included from a different data source, to check this you can search for Isodon forrestii in version 1 of The Plant List.