Unsourced names of mother for Hywel ap Cadell

Hywel ap Cadell's mother's name is, according to Medlands, unknown however this profile has been linked to two different mothers:
Rheingar
and

with no reliable sources to support the claims.
Can anyone please come up with such sources so we can decide whether to link one or other or neither of these as his mother.
Thanks
Terry (curator)