Hopefully it is a Norway maple, it does look like one, because they're less likely to poison horses according to this Cornell research article "We now expect not only red maple, but also sugar maple and silver maple leaves to poison horses. Similar analysis of Norway maple leaves indicates that poisoning due to the ingestion of leaves from this ornamental tree is much less likely."
http://www.ansci.cornell.edu/p...
I don't know if this is the latest info as I haven't kept up with it since we no longer have horses.
