So the general rule for garments worn over the lower half body, which you put your legs through separate routes, is that they are plural. We have plural-only nouns such as trousers, pants, panties, etc. But the word for the thong which appears to be a much skimpier version of the panties is singular. Why is that?
Its usage to indicate something similar to the lower part of a bikini is recent (1990). Probably the fact that it looks just like a narrow strip of cloth/leather as in its original sense is the reason why it is used in the singular:
Thong (n.) :
- Old English þwong, þwang "narrow strip of leather"* (used as a cord, band, strip, etc.), from Proto-Germanic *thwang- (source also of Old Norse þvengr), from PIE root *twengh- "to press in on, to restrain" (source also of Old English twengan "to pinch, squeeze"). As a kind of sandal, first attested 1965; as a kind of bikini briefs, 1990.
That is because we don't have trousers, pants, panties, etc… we have pairs of those things.
"A pair of trousers (or anything)" is a pair, which is singular and properly compared to singular thong, whether as Janey-come-lately lingerie or a traditional narrow strip of leather.
(While "thong" or "thong-type"are perfect terms to describe certain kinds of sandal, the thong is never more than a vital part of the sandal; it cannot be the sandal.)