[Everything in this paragraph seems like an overwrought version of Austin's bit about the finches that suddenly explode etc., and what we should say about them. I can't recall where that passage is. I need to read more Austin.]Well, everyone needs to read more Austin. Here's the quote from "Other Minds". Even in special cases (of deciding "whether it's real"), "two further conditions hold good": first, that it's not true that just "because I sometimes don't know or can't discover [e.g. because it flies away], I never can." The second is that "'Being sure it's real' is no more proof against miracles or outrages of nature than anything else is or, sub specie humanitatis, can be. If we have made sure it's a goldfinch, and then in the future it does something outrageous (explodes, quotes Mrs. Woolf, or what not), we don't say we were wrong to say it was a goldfinch, we don't know what to say." [Philosophical Papers, p. 88]
However, he also goes on to say that "It seems a serious mistake to suppose that language (or most language, language about real things) is 'predictive' in such a way that the future can always prove it wrong. What the future can always do, is to make us revise our ideas about goldfinches or real goldfinches or anything else." [88-89]
This is almost right, but it makes it sound like the case is asymmetrical: that the future can't always prove our beliefs false (rather than our "ideas"), but that our "ideas" are always vulnerable to (forced?) revision. What I would rather say is that both our beliefs and our meanings are corrigible, which nicely combines the ideas that a) beliefs are corrigible in the light of further experience, and b) the interconstitutive nature of belief and meaning implies that the same is true of meaning. While it may be natural in any one case to do one rather than the other, ultimately the choice is up to us. Neither "the world" on the one hand nor "language" on the other (as it seems some want to say) can determine our choice unilaterally.
I've always taken this to be the moral of Putnam's robot cat example. If those things turn out to be robots, then we have two choices: we can say either that a) the supposedly analytic and thus unrevisable sentence "cats are animals" has, mirabile dictu, turned out to be revisable after all, as cats have turned out not to be animals after all, but are in fact robots; or b) that "cats are animals" remains analytic, but that, mirabile dictu, it seems that there are no cats among us after all, as those things we thought were cats have turned out to be robots instead. I don't remember, but I think Putnam himself may have claimed that we must say (a) here, but it sounds better to say instead that we are not forced to say (b) (i.e. due to the incorrigible qua non-empirical analyticity of "cats are animals"), but can say what we like.
In general, my motto in such cases is that when something unutterably weird happens, it may be that whatever we say will sound unutterably weird, which means that examples like Swampman (or Twin Earth, or grue, or whatever) are nearly always not worth it – if there's really a point there (beyond what I just said), you can make it better some other way.