"Mae gynni hi wallt tywyll."

Translation: She has dark hair.

[sbnaidu]

Why do you mutate gwallt, and why do you say mae gynni hi ?

[EllisVaughan]

Firstly "gynni" is the feminine third person conjugated form of the preposition "gan" and it sort of means "with". So the sentence is "Brown hair is with her". This form of saying have is more common in the north whereas the south uses "gyda fi" more often. Secondly the rule I know for mutation here is that it's because "gwallt" is coming after a pronoun (in this case "hi").