P.S. The one major visible (there are others but they are not always present) difference between dun and buckskin horses is the presence of an "eel" or "dorsal" stripe. If the horse you saw had an eel stripe than she was definitally dun. If she didn't then she was buckskin.
Actually, this is not completely true. The smutty/sooty gene can create "countershading" that can mimic the dorsal stripe. Its not as clear as the dun gene, but it can still create confusion on telling a dun vs buckskin apart.
Its also possible to have a buckskin dun, so dorsal stripe can actually be evident.
The mane and tail are usually off-white (flaxen) but can have golden hairs or black hairs. Sometimes the sooty gene will create a 'gray' mane on a palomino.
This AQHA Mare (Northwind Gold, owned by Mikie Peebles) shows an unusual distribution of sootiness on a palomino. The mare's rear end has been darkened, and the barrel area dappled, but the neck shows her base color.
Ramblin' Lemon Twist -Missouri Fox Trotter stallion owned by Lee Yates. This photo shows the deep dappling that can occur on palominos.
The most beautiful horse I ever saw (color wise) was an appy. It had a dun colouring that bordered on palomino as the coat had this golden sheen. It was a blanket appy with a very straight blaze and black points on the ears and muzzle. The mane was black and the tail was black also and the haunch had these lovely golden/dun spots set on a white background.
All four legs had stockings then black at the knee/hock area then merged into the basic gold/dun body color. Could jump the moon and he was about 16:1 or 2 hh.
I have never seen anoth horse that had these markings after him.
As a stallion he never reproduced all of his stunning color points but many offspring had parts of him
Sounds like you are for sure asking about a Dun. As for the breed none of us can actually say for sure, as some breeds are prone to odd colorations as was pointed out in the arab. Also, Dun is a color and registration, so as for the color, it can happen in MOST breeds, with a few exceptions. The tail and mane color can happen, its all genetics...but the dark, prominate dorsal stripe is a tale-tale sign with little exception.