"Certainly not neuter!"
The difficulty is that nouns ending in -ēs were never neuter in Latin. In fact I don't even know if there is a neuter declension they'd fit into. So if this is neuter, it'd certainly be a one-of-a-kind exception.
Well, I just added "trapezoides" without any length. Someone else changed it into "trapezoidēs". Compared with other words ending in -oides the i should be long, that is "trapezoidēs" should be incorrect anyway.
Dictionaries mention several neuter nouns in -oides, like ascyroīdes, cynoīdes (or cynoīdēs), neuroīdes, sēsamoīdes - though some of them might be hapax legomena or words just used in science and not in poetry. In case of cynoīdes/cynoīdēs different dictionaries give different lengths for the e. But maybe there's no proof for any length. Also dictionaries mention adjectives in -oides, which are -oides for all genders in the nominative singular like monoides. But it could be monoīdēs, ēs, ēs (neuter with long e), or monoīdēs, ēs, es (neuter with short e like in Greek). There are also other adjectives in -es like diopetēs and īsoscelēs. But again the length of the neuter nominative singular might be doubtful, and maybe there's no proof for any length too and maybe it's just a guessing.
Based on dictionary entries it should be more likely that the neuter ends with a short e, e.g. for neuter nouns in -oides it's more often -oīdes in dictionaries than -oīdēs. That is, it should be trapezoīdes n (a and first e should be short like in τραπέζιον (trapézion) and τράπεζα (trápeza), plural should be trapezoīdea, maybe also trapezoīda). With accusative trapezoidem and plural trapezoides it should rather be trapezoīdēs m, but I have no Latin proof for the gender. Reasons to assume masculine instead of feminine gender: German Trapezoides with accusative Trapezoidem was masculine, and trapezoide and trapézoïde in Romance languages are masculine too.
As far as I know, trapezoīdes n is indeed a one-of-a-kind exception, but not because of its neuter gender, but because of the plural trapezoīdea which can be found in New Latin. For other words in -oides the neuter plural might be unknown, and in case of the adjective isosceles one can find the plural isoscelia. Side note: -oidea is also an inflected form of New Latin adjectives in -oideus like rhomboideus. That is, just finding a word form ending in -oidea regardless of context does not attest a neuter plural for a word in -oides. -80.133.101.204 13:53, 24 August 2016 (UTC)