Raanan Eichler, “What Kind of Creatures Are the Cherubim?” (2016), thetorah.com.
I believe that the the [sic] anonymous sage in Midrash Hagadol was correct, and the cherubim are winged humans. Recall that the ark cherubim in the temple are described as having a wingspan that equals their height (1 Kings 6:23–26). Not only four-legged animals, but also upright creatures, whether real ones such as birds or fantastic ones such as winged snakes, would have to have comically short wings in order for their wingspan to be only as great as their height.
Indeed, we don’t find any depictions of these creatures in the iconographic record that have these proportions. Only humans, who stand erect on long legs and are by far the tallest of any of the candidates in proportion to their other dimensions, can have wings whose span equals their height that still look respectably long.
The Amateur Exegete 