This name is a synonym of Crataegus monogyna f. pendula (Lodd. ex Loudon) Rehder.
The record derives from RJP
(data supplied on 2011-10-18)
which reports it as a synonym (record 18752) with original publication details: Dendrologie Dendrologie; 1869 160 1869.
This name record (from RJP) 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 Crataegus reginae in version 1 of The Plant List.