I believe this question means to imply that "wereman" was at some point in English history a conventional (or indeed the conventional) term by which an individual was attributed male gender (after the fashion of "man"). And this certainly was not the case.

"Wer" is very well attested as an Old English identifier of the male person (and especially, a husband). It is "wer and wif" god creates, in the Old English Genesis. And "wer" continues to see some use into the Early Middle English period (being defunct by Chaucer's time).

Moreover, various '-man' compounds are likewise well represented, through to Late Middle English. With OE "wæpnedmann" the one most oft mentioned among them, as designating male gender.

But as to "wereman", I can find no attestation of it, in any of the standard Old English and Middle English corpora, dictionaries and word lists I have available to me.

I would suggest that this compound was perhaps invented as an extremely probable-seeming, structurally analogous peer to "wifman", which just so happens not to in fact have existed, by whatever twist of linguistic fate.