I have heard the term 'Otter' and the term 'Viper' used to refer to the snake, but I have yet to see which is more used in the common tongue.
There aren't too many snakes in German speaking countries. The most common ones are
- die Kreuzotter (Vipera berus) – most known local snake
- die Aspisviper (Vipera aspis)
- die Äskulapnatter (Elaphe longissima)
- die Würfelnatter (Natrix tessellata)
- die Ringelnatter (Natrix natrix) – second most known local snake
- die Schlingnatter (Coronella austriaca)
- die Blindschleiche (Anguis fragilis), but that's not a snake – it just looks like one on first sight.
Exotic snakes are named unpredictably, but most likely as -viper rather than -otter if that matches. Die Kreuzotter would simply win for occurrences in German texts because it's the best known local snake.