Only 6 years late to this thread, but I grew up in Parkmerced - lived there from '59 thru '72 - and had to chuckle when I saw the questions about "Monkey Island" - my old stomping ground.
The story that I had heard was that it got it's name when a monkey, that had escaped from the SF Zoo, was found on the hillside above Brotherhood Way, swinging from tree to tree. The authorities captured it and took it back to the Zoo. Of course, I have nothing to support this story - it's just the tale that my friends and I picked up along the way. That the how nothing but "hobos" dared to stay in the woods there after dark.
But that sure was a fun place to take my "Johnny-7" toy machine gun and play "Army" with my friends! Does anyone remember the "cement slide" that was at the far west end, near the intersection of Brotherhood and Lake Merced Dr? I spent many hours there using a flattened cardboard box to slide down the hill.
Good, innocent, times...where have they gone?