They’re aquatic members of the order Cingulata (armoured new world mammals) and are related to the armadillo.  That explains the mammalian characteristics and the scales.

This would also mean that the brightly colored scales are the result of light scattering structures in the scales as opposed to pigment as no terrestrial vertebrates are capable of producing pigments other than brown melanin and yellow-y orange-y carotene.  Birds have structures in their feathers that refract light to create the appearance of color without pigment (also this is how blue eyed and green eyed people get their eye color)