Grains are going to be hot to varying degrees.
I don't feed grain to any of mine. What I use is copra (coconut with most of the oil extracted with heat and pressure). It's cool feed. Company that makes what I use is out of Australia. I've known some horses that don't take right to it, but they all eventually eat it (horses are big on change). The two I have now loved from day one. Mine get it soaked, but you can feed it dry too.
My current mare is an pretty easy keeper and she gets about 1/2 what my filly gets. My filly is an easy keeper too, but at 3 she's still growing more and tends to not put on as much weight as easily.
If you keep your horses stalled then I'm not sure I'd use it as a "complete" feed. It might need some suppliments. My graze, get varying amounts of hay (depending on the season) and don't need anything else beyond the copra.
Of course I also like it because it packs real well for transporting via horseback for training the horse for distance riding, but that's just a benefit for something I do and won't matter to most.
There may be other sources, but what I use is made by Stance, www.stanceequine.com
(smells like something I'd eat for breakfast) :))