I mean, say what you will, but a horse is likely to have a similar body temperature regardless their location on the planet. The same can't be said of boiling water. Pretty normal elevations make pretty big changes in water boiling temperature.

You can ultimately also average out temperatures for multiple horses to get pretty close to that 100F.

A better pin would have been something like the melting point of Gallium. Something that hardly changes with a change of pressure.