I dunno, Tranquill just looks too feminine for a pokémon with a 1:1 gender ratio (it does have a 1:1 ratio right? Don't feel like checking), just look at its "hair". By hair, I mean the crest of feathers at it's nape of course. Also, like Pidgeotto it looks kind of plain. Honestly I think the Pidgey line fairs so well because it was the original line.
Unfezant is just better. Well... male Unfezant is. Female Unfezant is just... unpleasant, but male Unfezant is badass!
And its shiny form is even more badass than its original form!
While shiny Tranquill is just... yeah...
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Nio slug bunny- http://img.imgcake.com/nio/60gennypngda.png
Raka anime chick- http://img.imgcake.com/Rheine/Gennypngmu.png