Welsh mutations
These occur when the mutated letter follows particular other letters

Examples

Coch = red (like Byrn Coch, where I live - which is Red Hill)
Draig = dragon

red dragon becomes
y ddraig goch

Other possibilities include
B -> F ( as in Bach -> Fach (small))

C -> G -> Ng

D -> dd (pron Th)

Complicates the dictionary somewhat when the first letter of a word can change.