Q:

When did Korrigans become part of the Breton folklore?

Korrigans are part of the modern Breton folklore, as water spirits. Some sources give them an origin in ancient Gaul (Thomas Keightley's 1892 "The Fairy Mythology" for instance). Is there any evidence for this?
From some of the stories surrounding those creatures, many seems related to the rise of Christianity (see the ones related on the wikipedia page for instance), hence the more natural idea that the Korrigans legends actually originated later.

A:

This compares the Korrigans to three earlier mythological figures:

Ceridwen
The Lady of the Fountain
The Lady of the Lake

Ceridwen is first (and perhaps only) mentioned in the story of Taliesin, in the Mabinogion. She is not depicted favorably - as a magical figure who, to put it one way, reacts poorly to failure. She really only appears in the first part of the story; her role leads to the life of Taliesin, the main character. her name, however, sounds quite similar to that of the Korrigans. Given that mythological names can change over time, she could be an important precursor.
The Lady of the Fountain is a tale of King Arthur, also related to a French poem, Ywain, or, The Knight with the Lion. It features (of course) the Lady of the Fountain, also referred to as the Countess of the Fountain. She does not appear to directly have any magical powers, but the Fountain does - at least in the story of Ywain. In this tale, she is named Laudine. The important tie here is that in the French version (as well, I think as the Arthurian), the Lady hails from Gaul or Brittany.
The Lady of the Lake is, of course, a central character in the stories of King Arthur (though there are some odd discrepancies; I would be indebted to anyone who can answer this). I think that while the etymological argument in favor of Ceridwen being the influence is strong, there is also a very good argument to be made for the Lady of the Lake in Avalon. She is described as having eight other sisters; this is quite consistent with the nine Korrigans Keightley refers to as existing on some versions of the myth.
Each character has one or more chief similarities to the Korrigans, aside from any magical powers and other defining characteristics. For Ceridwen, it is the name. For the Lady of the Fountain, it is that she hails from Gaul or Brittany. For the Lady of the Lake, it is that she is one of nine sisters. It does seem likely that the Korrigans are composites of the three, meaning that the Korrigans did not appear in Breton folklore at a particular time, but were corruptions of earlier tales.

