“When we got the master plan for the park approved in 2000, we called ourselves the Friends of the Greenbelt, like many “Friends of ….” organizations, and the park was called the Greenbelt or the Rockridge/Temescal Park or sometimes the Creekside Greenbelt Park. At a meeting in mid-2000, we were talking about the creek and aquatic life that would be in the creek if it flowed year-round, such as frogs and salamanders. Because nowadays everything seems to become an acronym, a few of us used “FOG” as an abbreviation for Friends of the Greenbelt.”  Someone (I don’t remember who) said “instead of FOG, which sounds like San Francisco, maybe we should be FROG, friends of the Rockridge Greenbelt, for the creek and the frogs” and it was like a flash of lightning. We altered the name slightly to Friends of the Rockridge/Temescal Greenbelt, and became FROG at that time. As we continued to work on plans for the park, I believe it was our Big Build co-chairs, Eileen Fitz-Faulkner and Steve Costa, who suggested we stamp a frog design in the pathway, and then that we call this “Frog Park” instead of the Greenbelt. It made perfect sense, given the creek and the theme of the park, and throughout 2001 when we were recruiting volunteers for the project we started to call it Frog Park. So it actually started with the acronym for the organizing group, and then the name Frog Park came about less than a year later. Eventually (it took some years) the city adopted Frog Park as one of the official names of the park, and the names Hardy Park and Redondo Park were retained (this has to do with what is listed on the police and fire databases, though few people use those names nowadays). A few years ago, we started calling the Hardy Park play structure area “Big Frog” and the Redondo play structure area “Little Frog,” which seems to have resonated with people.” — As told by Theresa Nelson, Frog Park’s co-founder

Here’s a summary of the park on the Oakland Wiki.