After doing a bit of research, I am reasonably certain that the "odd frog" is an hourglass frog (Dendropsophus ebraccatus) without the usual markings. Here is what they usually look like:
I photographed this frog quite near where the "odd frog" was, but notice the roughly hourglass shaped dark blotch on its back. It would seem, however, that the frog can come in just a plain yellow without the blotches, just much less commonly.
Keith Christenson - Wildlife Biologist