Because he is considered to have a special place in Welsh religious history, he is known in Welsh as the equivalent of 'Dewi the Saint' (rather than just 'St Dewi').
This is quite common in the titles given to particular historical figures. For example:
- Hywel Dda - Hywel the Good (sometimes the honorific is mutated, as here)
- Rhodri Mawr - Rhodri the Great
In the previous question, translating from English, I put "santes" for "saint" and was marked wrong. Some inconsistency here?