Does every object in JS have a toString() method?
No. Only those that inherit it from Object.prototype (as all normal objects do) or define it on its own (or inherit it from their custom prototype) do.
You can create such unusual objects by Object.create(null). You also could give a plain object an own toString property that shadows the inherited one and is not a function (e.g. {toString:0}), but I guess that would’ve throw a distinct error.
In your case, it seems that the querystring parser used by bodyParser() does (did) indeed create objects without prototypes, to avoid mangling .constructor.prototype when such parameters were used. See qs pullrequest #58 and express issue 1636: Bodyparser not setting object.prototype? (suggesting an update).
How to reassign a proper toString method?
You could just assign any function, like
req.body.toString = function() { return "Hi, I'm a request body"; };
but probably you want the standard one:
req.body.toString = Object.prototype.toString;
Other options would be redefining the prototype via the non-standard __proto__ property (req.body.__proto__ = Object.prototype) or simply applying a standalone function on the object instead of making it a method, like Object.prototype.toString.call(req.body).