From Dragon Quest Wiki

Tonnura (トンヌラ) was one of the possible names for the Prince of Sumaltria in the Japanese version of Dragon Quest II. The name Tonnura has become an in-joke in Dragon Quest ever since. To start with, the name sounds "odd and stupid" to Japanese people. On top of that though, in the Famicom Version, the Prince of Sumaltria is kind of a weak character. Between these two facts the name Tonnura became known as being a particularly weak and uncool name, but not a hated name. The impact that this name created has caused the name to reappear in the Dragon Quest series on multiple occasions:

Wikipedia says that Yuji Horii came up with the name Tonuura from one of the fantasy books that he was using as a reference. From that they think that he is named after Ernest Tonnelat.

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