There are six minis that are considered to be "official" alternates for Talisman as Games Workshop used to send these out in place of other minis when they were running low on supplies of the standard ones. These are the Elf, Knight, Samurai, Warrior, Warrior Of Chaos and Witch Doctor. They have even been known to turn up in some of the boxed sets.
A recent addition to this page is the alternate Assassin figure, which appears to have only been sent out in boxed sets of miniatures in America. I have now seen a few examples of this and so am adding it to the list as an "official alternate"!!
The Champion Of Chaos figure that is presented here appeared in my collection when I bought them as a job lot. It was said to be sent out by Games Workshop after the publication of White Dwarf #115, though I have never seen this mentioned elsewhere.