There are 2 points to consider here.
1) Fat feeding in general. There is huge controversy about feeding fat to horses, with equine nutritionists sharply divided on the subject. However, even equine nutritionists who are opposed to it in working horses will usually suggest it for horses which need to put on weight.
I have had excellent results with rescue horses with Rice Bran Oil and Equi-Jewel.
If you do feed oil, you need to consider the following.
2) If you are feeding fat, wherever in pre-mixed feed or as sunflower oil or any type of oil, you need to increase the protein in the horse’s diet, and make sure that the horse is eating plenty of hay and / or grass. It is very important that the horse is not eating fat at the expense or hay or grass.
Another very important point to consider is the Omega 3- Omega 6 ratio. It is important to get Omega 3 into the horse. Omega 6 is pro-inflammatory.
These have ZERO Omega 3 (remember, you need Omega 3) and lots of Omega 6: sunflower seeds, sunflower oil, rice bran, rice bran oil, cottonseed meal, peanut meal, and safflower meal.
If you would like to feed these oils, you can (must!!) get Omega 3 into your horse by feeding linseed seed (called flax, flaxseed in the USA – exactly same thing). Linseed/flax is optimally balanced for Omega 3 and Omega 6 for the horse. You can feed up to around 450 grams ( 1 pound) per a day. You will need to grind it in coffee grinder. Do NOT boil it as this kills the Omega 3s. I also do not advise to buy the flax oil /linseed oil – not only is it far more expensive, it is unstable and needs to be refrigerated.
On the subject of Omega 3- grass is also supposed to be high in Omega 3, but I myself am not happy about what is “supposed” to be in grass. “Grass is “supposed” to be very high in lysine, but studies have shown that tropical grasses are in fact low in lysine.
Some advertisements say that chia is higher in Omega 3 than flax/linseed, but I have not been able to find a study which states that this is in fact the case. Chia is also hugely more expensive than linseed. I keep a little on hand for those times when I just cannot face to do another day’s grinding of the linseed/flax!
There is also some controversy about Linseed/Flax being possibly poisonous. A non-nutritionist scientist has advised not to feed it, but leading equine nutritionist Dr. Eleanor Kellon has advised to feed it. There has been no case of human illness or poisoning from fax/linseed.
You really do need to get the good Omega 3 of flaxseed (linseed) into the horse if you are feeding these oils or oil seeds, or premixed feeds with these in.
As I said, I often use Rice Bran Oil (which sadly is more expensive than Canola Oil -which does have some Omega 3 – which I also use). I feed around 110 – 120 grams (3-4 ounces) of Rice Bran Oil per day, sometimes a bit more.
One more consideration – many commercial oils for horse consumption are fish oil based. I myself would not give fish oil to a horse.