This is a tricky question, because Bree and Hwin are not really described in much detail, aside from the fact that Bree is a warhorse and Hwin is a brood mare. Using this as a starting point, I would say that Bree should be a fairly sizable horse, perhaps a stallion? He should also be fairly muscular, but not overly so. The environment of Calormen, and perhaps the treatment by some previous owners, may have worn him down, and his physical looks should reflect that. As for Hwin, I think she should be smaller that Bree, but not so much that it creates a kind of superior-inferior dynamic between the two horses. She should also be more graceful and perhaps gallop a bit more lightly. The color of the horses is perhaps the biggest challenge, since it is never mentioned. In the colored edition of the Chronicles, Pauline Baynes showed Bree as white with gray speckles, and Hwin as a kind of pale chestnut. Those would be the ideal colors for me, but I'm open to other interpretations as well.

Bree should be a dappled grey and hwin a bay!

Hwin was in my mind with grey speckles (I didn't have the color version) but I like the idea of Hwin being a bay. I don't know to much about horses, though.

Here are two pictures of horses that I think could be Bree and Hwin. Let me know what y'all think.

aslansothername, I really like the colouring on those horses! That's pretty close to what I imagined. But I think I see Bree as a little heavier and more powerful; I reckon that's a side-effect of Baynes' illustrations, though.

I never realised that their colouring isn't mentioned - and I just re-read "The Horse and His Boy", too! That shows how much I rely on those colour pictures.

@ aslansothername: those look great! However, personally I would prefer Whin to be more of a palomino.

Just out curiosity do you know size of the "Whin horse". It looks to me to be a little big for Whin.