There's a video on the internet of a dog under the covers and the owner lifts the covers as the large dog spills out and onto the floor like water. The person refers to the dog as "she".
In one of the replies, it read: "Why would you do that? they were so cozy"
I looked at the video again, expecting to find two dogs or a dog and a cat, thinking I missed the point of the video, and asked about it. I was told it's the singular version of 'they' that has been in use since the 1300s.
Is this the correct usage?