what kind of tree?

I don't know, OP, but would have to dig into my tree guides for that. The photo showing the long (eliptical) leaves resemble maple seeds/ash leaves.... the website you posted was rather puzzling, as I've never seen maple leaves identified as that. I would suggest a good "Trees Of Canada" field guide in the nature section of your local library, rather than the web. Good luck :)

This website says ash trees have similar seeds...
Arbopedia | Arbopals I'll double check the pasture just to make sure.
Last year we had my horse in that paddock and there were lots of leaves on the grass but Duke didnt get sick. Does that mean that those trees arent toxic?
Thanks everyone!