I’ve been in contact with someone who thinks she knows who Dexter is. She described his markings pretty accurately (left shoulder marking of a longhorn, and another similar shaped marking under his throatlatch which you can not see in the pictures but he does indeed have), so it sounds like it is probably the same horse. If it is him, he would be a bit older than 3, more like 4 or 5 years old. I’m having the vet out to give him his vaccination boosters on Thursday and going to have her take a really close look at his teeth and try to narrow it down.
The other horse was named “Tex” (ironically very close to Dexter) and was purchased as a 2 1/2–3 year old a year and a half ago at the Everson Auction. He was a stallion at that time and was gelded and started under saddle as a roping horse, and he was indeed unflappable and laid back then as well. He was then sold when the owner was injured by another horse of hers and could not ride. Her friend recognized his photo on a flyer that Emily Diaz had put up in Skagit County and contacted the county.
Physically, he still looks to me like a younger horse in a growth spurt as he has no withers and is quite butt-high at the moment (I sticked him last night at 14.2 at the wither, 14.3 at the hip). But the older age would explain his training. My guess now is he is about 4 — which would give both vets a margin of error of 1 year which is plausible.