I'm sure there is some earlier game that could be classified as Masocore that I'm not thinking of, but at the very least it seems to be the first indie game to be classified as such.
Don't worry about which was first. Just add any games that fit the concept. When the new site launches, the first game will be chosen automatically based on release date.
This blog by Anna Anthropy seems to have popularized the term, though it was apparently coined by someone else in a forum earlier. She cites Super Mario Bros. 2 (The Lost Levels) as masocore, and that one predates Takeshi's Challenge by nine months.