Temmie is a species of monster in the Underground. They appear as a unique enemy in Waterfall, residents of Temmie Village, and the vendor of Tem Shop. Contrary to popular belief, killing Temmie accrues EXP and is considered a kill by the game, which will cause a True Pacifist Route attempt to fail.

Just make Temmie sound like Timber Puppers' video on it, here's an example: https://www.youtube.com/watch?v=YPwfUYDSXPY