Many auto-installers choose "zen_". Doesn't matter what *they* selected. The key is what EXACTLY is ALREADY in YOUR old files ... since that's the prefix that was used to access the tables ALREADY in the database you exported.

In a perfect world, no prefix would be used, since prefixes only cause confusion. Sadly, cheap hosting companies which offer only one database make the use of prefixes required in those isolated cases. Thus the option exists.