Sorry, flowers are the common example.
I think an easy one would be the BEL complex. When you breed a super lesser BEL to a normal you get hatchlings that have one BEL allele and one normal allele. Lessers are significantly lighter than normals but are not as white as BEL. They are also evenly lighter all over their body (red+white=pink, incomplete dominance). If it was a codominant situation, the hatchlings would look very similar to pieds. They would have sections of pure white and sections of normal coloration, but no intermediate color (white+red=large splotches of both colors, co dominance).
The same is true with super pastels, enchis, spotnose, black pastels and all the other genes we commonly call codom.....in all cases the animals with a single allele look different from both the super and normals, having an intermediate look. If all those genes were codoms, we would have a lot of animals looking like they were splashed with different colors of paint.
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