A Shiny Pokémon (Japanese: 色違い differently colored or 光る shining), also recognized as alternate coloration or rare coloration, is a specific Pokémon with different coloration to what is usual for its species.
--Bulbapedia
So, it's just different coloring, not an alternate form which is
While there are 649 officially recognized species of Pokémon, with most members of an individual species looking relatively similar to one another, several Pokémon species have remarkable differences in appearance between members. At times, these differences are purely cosmetic, and have no bearing on the difference in the Pokémon's stats from another; however, several Pokémon not only differ in stats, but also type, depending on their form.
--Again, Bulbapedia
So you see there are no remarkable differences, just alternate coloring, hence, shinies not shown as "another form" in the pokedex.
Example : 
and 
There's just color difference here, whereas
V/S. 
and 
There is a lot of difference, in body and form; Hence the first case show Shiny Pokemon, and the Second, alternate forms.
Hope This Helps :)
