Spelled without the final sigma in several manuscripts (and in ps-Zonaras), which would suggest that the present headword is genitive; but either way, unattested outside lexicography.
There are however [NF] two rivers called Himeras - the present-day Grande and Salso - in Sicily (Barrington Atlas map 47 grid D3/4; the Grande flows north to Himera, iota 345). There is also a river Himeros near Chalkedon, in present-day Turkey (map 53 grid B2).