Aderyn is bird and is masculine. Deryn is how people usually say Aderyn in speech. It's not really a name.
If I said someone was 'yn dipyn o dderyn' (a bit of a 'deryn/aderyn')I'd be saying he's 'a bit of a player'
The names like Melys, Cariad and Deryn look like the person naming the child didn't understand Welsh well enough to get the nuance.