Both seem to be referred to as terfysgwyr in the dictionary. Is that correct that one name covers all? You could argue there’s a bit of a difference.
GPC does give both rioter and terrorist as definitions for terfysgwr, but GyA differs - it says terfysgwr is rioter only, and that terrorist is brawychwr or dychrynwr
The GPC also lists rhawter and brythai for rioter. And in addition to what dictionaries may say, author John Alwyn Griffiths (who used to work as a policeman) has used terfysgwr as terrorist in one of his novels.
dychrynwr feels a bit makey-uppy.
brawychwr, however, makes a lot of sense.
Just my personal prejudice, though. 
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dychrynwr sounds like someone going “cast a cheiniog” … trick or treating haha