Perhaps severa's is the lump arenvald mentioned: its not a true third eye but its certainly there and obvious enough.

With half breeds OP the result generally takes more after the mother's race. This is why hilda is closer to a hyur than an elezen. Similar thing with the au ra from legend, her child had less scales than usual.

Its known that each of the base 5 races can have offspring together, and with au ra that seems to continue. Unknown on viera and hrothgar.
Regarding horns, given genes sometimes skip a generation and sometimes dont, there's probably no consistent basis on which horn types an au ra gets.