No sooner I finished a post here than it occurred to me that there is an expression which seems to be related: "enough to make a cow laugh". Everyone knows that cows do not laugh and the idea of an animal laughing which usually appears to be exceedingly grave, or maybe bored, gives the idea that it would take an unbelievable degree of funniness. Same principle, I think in choking a horse.

I'm sure there are lots of other expressions of this kind. Who will start listing them? I'll start with: {Disgusting] enough to gag a maggot.