I first thought that she may have had issues because she didn't grow although her siblings thrived. Then when she didn't develop feathering on her head I though possible hybrid. Now I'm confused! She seems happy enough so I guess only time will tell!
She may very well be a hybrid..I think that is quite likely. It would explain a lot.
I'm not up on my guinea/chicken genetics but usually hybrids that are not common are not common for a reason...ie they have less than successful offspring.
Guinea/Chicken is not a common hybrid apparently for good reason. The hybrid is reportedly always sterile (common side-effect of cross species offspring), and one article I read stated they are relatively short-lived even with a directed breeding program.
http://www.feathersite.com/Poultry/Guineas/BRKGuinHybrid.html
Again, the curse of the cross species offspring.
One well known and usable cross species hybrid is a mule (donkey/horse). While it is generally a "happy" hybrid (as in healthy and strong...some mules are rather cantankerous)...still the mule will always be infertile. (I think there is only a handful of recorded cases of a mule actually having offspring since 1500's?)
Less common hybrids happen, but usually are genetically are weak and suffer from multiple medical issues because the genes just are meshing that well.
So if there are no naked necks on the farm, and the potential for guinea/chicken hybrid, that likely may have happened. It sure looks like it. And why she/he is not growing well.
Lady of McCamely
EDITED TO ADD: you've piqued my curiosity, so I googled and surfed a bit and found this article
http://www.aviculture-europe.nl/nummers/08E06A07.pdf
Interestingly Guinea/chicken hybrids have been done for meat purposes, but almost always the offspring is infertile. Gives me further reason to believe your likely Guinea/Chicken has some internal issues going on as it should be much larger than it is.