For green fire read Greek fire
For Mammoths read shaggy elephants (they're cold bless em!)
For Giants read... very large people(?)
For Dragons read... Dragons
For undead read...unstoppable levy reinforcements
For Others read...Others
OK I'll grant you the last few have little or no historical equivalents, but do any of you lot recall Wargames Research Group's attempt to do a Lord of the Rings adaptation of their early WRG rules called 'The Lidless Eye'? Such things are doable with a little creativity.
Captain Blood is right. This is an excellent framework around which to accumulate Pseudo-Historical shiny new toys to game with. I'm having a hell of a fun time converting barbarians, celts, saxons and vikings into Wildlings and Ironborn and Feudals, HYW and WOTR into bannermen. And yes I do have a Giant and a mammoth.