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.
Words brought to Japan from English speakers and which end in 'y' are usually pronounced with extended 'i' sound.
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. :)