Ah, how'd I miss this one! Lol, t'is a beautiful river.
Hmmm. I reckon what you probably need is the local Welsh names surrounding it and further up towards the source - do a bit of antiquarian etymological pondering. Can find some amazing things out about the mythos of the Welsh landscape from that.
There's a little bit
here on
British Goblins.
There's also an
interesting thread about the name:
"Later the name evolved into Caer Dydd. The fort was built next to the river Taff, and there are some who think the name of the river and the name of the Roman have been confused and ended up being blended together to get the ff ending in english. "
"Caerdyf, a form which dates to the early 6th century at least, contains two place-name elements. The first is Welsh caer, 'fort, stronghold, enclosure' which is often seen in Welsh place names. This is followed by Taf, a river-name in its genitive form Tyf, mutated to Dyf - 'fort (of the river) Taf'."
Marie Trevelyan is usually the first pace I go for anything on Welsh folklore. Look under chapter one on seas, rivers and lakes - there's some info but it won't be extensive unless you can work out the name originally given as Taff/Taf appears to be quite a bit later.
Best wishes, if you find anything please report back!