But we have rat to

hemlo

So I initially thought she was just another blue in the litter as a pup.

Except she couldn't seem to make up her mind which of her siblings she looked like.

Was hard the make out the line as a wee fuzzy, but the pattern is on her skin too, not just the fur.
She is a chimera, the result of two embryos fusing together in the womb to become one rat.

A few days later, you can see the line more clearly as fur starts to come in.