Gold Bird

It is a normal bird wearing a gold coat of paint, and the substitution happens rarely enough that you will not find one by looking. A Lifeform Analyzer is the practical answer, since it will flag the bird as unusual while it is still on screen. Move fast when it does: the thing has almost no health, other enemies will kill it, and a stray swing from you finishes it just as well.

Two honest options. Sell it, which early on is a serious lump of money for one inventory slot, or cage it. A Gold Bird Cage is the display version and it is a one way trip, since a caged critter only comes back out through Shimmer. It also cooks into Golden Delight if you want the food buff more than the money or the decoration.