Sarcasm? Of course, we know this has happened, and somewhat often, in the past. However, in minor defense of peanut allergy sufferers, peanut allergies are, on average, much worse [potentially deadlier] than dog allergies and also more easily triggered. But yes, I agree with the premise that those with special needs need to bear the burden of those needs, including whatever pre-planning is necessary (epipens, masks, whatever).

But, this is a forum for pets, not peanuts...

Quite true, and this is a major issue. Some airlines actually do have carriers available for sale, though, for a similar reason... some people just don't bring the proper carriers and airlines can make some minor revenue by selling them. This may more for in-cabin pets, since some people will just carry them in a handbag, whereas most people checking their pets will have the proper container, though. In general, however, I believe the carriers are only for sale outside security, i.e. at the ticketing/check-in area, and not at the gate, which is where this kind of exchange ("I'm allergic;" "You must check your dog") would be happening.