The Japanese word for "Guinea Pig" is morumotto モルモット, a katakanization of "Marmot."
Another word for "Guinea Pig" is tenjiku-nezumi 天竺鼠.
Scientific name: Cavia.
Beware: some kanji that are difficult to handwrite are easy to type on the computer, which may skew the usage statistics.
Wikipedia: Guinea_pig (モルモット)