This might be in the wrong place, (writer's or opinions board might have been a better choice), but some names that I think could work are
Dark meanings-
Melanie,
Nila (or change spelling to Nyla),
Kali (or change spelling to
Callie),
Li (or change spelling to
Lee or
Leigh),
Nigella,
Maura, Mauricia
Horse meanings-
Phillippa,
Rosalind,
Rosamund,
Roswitha (hros was ancient Germanic for horse)
Adela could mean secretive, so maybe that would work, too.
Nyla
RoswithaCallie Phillippa
Mauricia
Rosalinda