No, you weren't hallucinating if you saw four goats in the Arnold Arboretum down on Peters Hill. The goats are being "employed" in a pilot program to control invasive plants -- by eating them.
Goats are known for being voracious eaters and will eat noxious plants like Japanese bittersweet, poison ivy and buckthorn. The goats are enclosed in a sizable electrified fence, which also protects them from predators, as well as dogs (keep those dogs leashed in the arboretum!).
They also have a shelter so they can have some privacy and get away from rain. The goats and their shelter will be moved around the arboretum as they eat their way through areas of invasive plants.
If the pilot program is a success it may be expanded in the future, which could in turn lessen the use of chemicals and reduce mowing.