I have a young 6 year-old arabian mare thats need to gain a bit of weight. I read in the latest Practical Horseman issue that adding a little corn oil to a horse's feed can help then gain weight, but what about canola oil? is it safe for horses and does it cause the same effect? 

I believe we've used canola.  Especially if it had been on sale.  We've also used peanut and soybean.  The horses' favorite so far has been soybean. 

We have 4 arabians.  One of the geldings is a very hard keeper.  Aside from vegetable oil, he gets beet pulp, soy meal, grains and supplements.  When there are mares around, he assign himself the herd stallion role.  His vigil causes him to tense up and burn calories while standing guard.  Now that we've release all the mares, he's beginning to gain and hold. 

Canola oil is fine to use. At the barn that I work at there are 3 horses (boarders) that are on canola. The canola also seems to help give the coats a nice shine as a bonus! Rice bran is another option. With some horses, both are needed. After a few weeks, we will then up the amount of hay.

 Thats cool, thanks to you both!

I wouldn't suggest corn oil, but rather ground flax, shredded beet pulp that has been soaked and/or rice bran and if you want to add some crimped oats.  My horse lost a lot of weight due to poor hay in the winter.  I started him on the beet pulp, increased his crimped oats, and feed HP1 and he put on about 100 lbs.