We generally don't use the term "oligarch" to refer to americans, even when the definition does fit. It's usually applied to the super-wealthy shadow-ruling class in eastern Europe.

Most of the mansions were built in the gilded age - 1870-1900ish range - so closer to 100 than 200 years ago. But they were built to mimic historical styles so they seem older than they actually are.

All of them are competently landscaped, but it sounds like you might be thinking of the Green Animals Topiary Garden which is delightfully over the top, but is also now closed for the season.