A lot of things seem off about Venomoth. While it has its comparisons to Caterpie and Metapod, Venomoth has more striking comparison to Venonat IMO (e.g. the color scheme and fangs).

What also throws me off is that Venomoth actually does resemble a regular moth instead of a butterfly. Butterflies are usually indicated with bright and outstanding colors, but Venomoth's color scheme is dull and monotonous.

People were probably making comparisons between Venomoth and Caterpie+Metapod just because they thought Butterfree and Venonat were too similar to each other (who actually have striking comparisons, compared to the other group).

But I think that Venomoth was intended to be Venonat's evolution all along, but that it was made after Venonat and the entire Caterpie line (as in, someone made Venonat and Butterfree first, but then someone else came up with Caterpie and Metapod, and it was decided that Butterfree would be the evolution of Metapod, meaning Venonat needed a new evolution. So Venomoth was created). This theory seems a lot more realistic than this simple switcharoo.

Not so sure about that. Caterpie is clearly a caterpillar, Butterfree is clearly a butterfly, Venonat is clearly a bagworm, and Venomoth is clearly a moth. Caterpillars metamorph into butterflies, just as bagworms do into moths. It was merely a coincidence that Venonat and Butterfree look similar.