Is it grammatical to say "the batmen"?

Solution 1:

I would say that strictly speaking it is not. Irregular plurals carry if they are instances of the base word. A "fireman" is a type of man, so "firemen" is the appropriate plural, but the "Toronto Maple Leafs" are not leaves.

Since Batman is a proper noun, Batman does not designate a type of man but the name for one particular man, so the irregular plural does not carry. It should be "the Batmans". However, this is, at least to my ear, somewhat awkward.

So you have three choices. If you want to be strictly correct and not be awkward, "the actors who portrayed Batman". If you don't mind being a bit awkward, "the Batmans". If you can tolerate a few points off on your poetic license, and can see Batman as not so much a proper noun but as a type of man, then why not ... "the Batmen".

Solution 2:

You could write the batmen, and if you’re writing an attention-getting headline, maybe you should. But it is definitely unusual enough to call attention to itself, and you typically don’t want to distract the reader from what you’re trying to say.

Batmans sounds obviously wrong to me, unless you’re going for laughs. Cf. librarymans.

For what it’s worth, there is an actual episode of an actual Batman-themed TV show titled “Night of the Batmen!”. Apparently the Batmen of All Nations are a thing too.

Maybe it’s just me but it seems like real sentences using batmen tend to be a bit informal in tone:

The result was 3 pages of Batmen in their off-hours, when they’re just kicking back and taking a break from being the Batmen we all know and love.

while those using batmans are extremely informal in tone:

I freely admit I love ALL THE BATMANS.

i think the batmans weapons cannot even harm iron man as he withstands powerful missiles.so ironman wins the game.

All “decent” actors when giving the approriate character… but not in this type of movie…. maybe in the batmans of the 90’s…