The reports of the so-called Mothman in the late 1960s, mostly from West Virginia, are generally presented as a consistent picture of a shadowy man-like figure, seven feet tall, with glowing red eyes and a ten-foot wingspan. As a matter of fact, most sightings are not so definitive nor yet are they that consistent. For one thing the reported size of the featureless shadowy figure varies a great deal from a short-human size to the larger-than-human size, and actually the reports in the shorter-than-average category are more numerous by direct count. More importantly, Mothman reports consistently say that witnesses could not distinguish either a head or feet. So basically nobody knows where the thing starts or where it ends really.
Mark A Hall has suggested that Mothman is a giant owl or what he calls "Bighoot". This is especially because the reported red eyeglow matches owls and in owls it may be difficult to distinguish between the head and the torso. In this case there is no reason to erect a whole new species, the sightings could refer to the same individual creature and one of exceptional size among its own kind.
The typical satance when Mothman is seen seems to correspond to a typical bluffing stance among the owls wherein the wings are held up above the head in order to make the owl look larger than it really is, and thus more intimidating. Therefore the 'height' estimates do not need to refer to the actual height but would instead refer to the height of the wings being held above the head.
I am willing to say from the reports that Mothman might be as much as three-and-a-half to four-and-a-half feet high with a wingspan of six or seven feet, maybe a little more. And it is not necessary to say they need to be that large if the typically teenaged witnesses were exaggerating out of fear or in order to match the previously published accounts.
Therefore I propose that sightings of Mothman refer only to an outsized great grey owl down from Canada. Great grey owls can have a body a yard long and a wingspan of five feet, going by Wikipedia. A gigantic one at half again those dimensions would be right in the range of those size estimates above.
Along with this is the possibility that the Cornish Owlman was another of these birds given that the descriptions are similar to Mothman (as reported by teenaged girls again)
There are a lot of other features about Mothman sightings that are probably inaccurate observations, assumptions going on facts not in evidence, and clearly mistaken associations with unrelated events. One case where Mothman was assumed to be flying overhead of a car going a hundred miles an hour could be nothing more extraordinary thar a squeaky motor noise. And there is certainly no reason to connect sightings of any big bird to any collapsing bridge.
As a footnote to the last matter, I have gone down to the pub at the corner of my block and heard a fellow that was in West Virginia at the time tell about the collapse of that bridge around Christmas time. He is completely scornful when the name of Mothman was even mentioned.