I'm afraid I'm not very familiar with the climate in Ottawa! I believe it can get quite cold? I'd probably rug her, as foals less than a year old tend to have a harder time maintaining heat. However, you would have to keep a close eye on her as an active foal is far more likely to get tangled up in a rug.
In saying that, I do know in a major stud farm in Japan, their yearlings are turned out without a rug, even though the winters are pretty severe. However, it's a dry cold, and not a damp cold. In Ireland, it doesn't get anywhere near as cold, but due to the level of rainfall, moisture and wind, horses have to be rugged a lot sooner (though we still probably over rug).
Hi Caoimhe, sorry, you are correct; I should have provided more info. I’m in Ottawa, ON. The foal is a thoroughbred/curly cross. She is kept outdoors with access to a shelter that at one time was a log cabin.
This really depends on a number of factors. What breed is the foal? Where in the world are you? What's the weather like? What facilities do you have? This would be a difficult call for most people to make.
However, in general, I wouldn't bother rugging a foal, unless it becomes clear she needs one.