Then Amanda at The $900 Facebook Pony posted that her Henry is April Ludgate and I figured it out!
Tristan is Ron Swanson.
He is the most stoic horse in the barn.
He has a really extraordinary capability for completely zoning out and ignoring everything around him.
Food is his reason for living.
Everything, literally everything, has to be his idea first.
This is his first reaction to literally every question I ever ask him.
He is really independent to the point of almost actively antisocial in the field.
But when he settles in and decides on something, he is ON.