Though scales area present in vertebrates of almost all varieties, from sharks to mammals, there are are acturally two types of scales with very different compositions and developmental origins.
Dermal scales have their origin in the dermis beneath the ectoderm of the embryo and are thought to be largely of neural crest or dermatome origin.
Epidermal scales have their origin in the ectoderm. The way in which they develop is more similar to hair and feathers. So, epidermal scales are analogous to dermal scales and homologous to hair and feathers.