"I want an owl."
Translation:Dw i eisiau gwdihŵ.
- Mae Siôn yn moyn banana - Siôn wants a banana
- Mae Siôn (ag) eisiau banana - Siôn wants a banana
So, several ways to express Siôn's desire for a banana.
moyn is a verb-noun here meaning 'wanting, to want', a common contraction of mofyn, ymofyn, which has a few other meanings, too. The eisiau option is used all over Wales with various pronunciations, while moyn crops up mainly in various parts of south and south-east Wales.
Since it is a verb-noun, yn is required with moyn.