In Japanese, using Katakana, why is the name Danny written and pronounced ダニー. Why not just ダニ? I'm not sure why the
i is being extended when this doesn't really happen in english.
To build on koji's answer, sometimes you'll see changes in the katakana-ization of foreign words and names to avoid collisions with native words and names. An additional incentive for the long "y" sound is that there's a native word だに, which means "mite or louse" (that is, a small blood-sucking arthropod). It's probably a *good* thing that the katakana pronunciation of "Danny" has shifted the way it has. :)