Hah — I had to look it up to double-check, but "Mori no Kuma-san" is indeed the Japanese translation of "The Other Day I Met a Bear", a great old classic camp song written in 1919. (It's the same tune as "Sipping Cider" or "Princess Pat", if that's more familiar.) The Japanese lyrics go back to 1972 — and it's the song sung by the Kuno siblings on the Ranma 1/2 "Karaoke Battle" semi-drama CD.