Welcome to Librarium Online!
does ironback boar work on chariots, if so does it count as an extra boar or replace one of them or both of them?
No, you can't. Sorry.
http://www.armyroster.com Check out my ToP WIP in the projects section!
Reason: You buy stuff for the character, not his toys. It's like giving a Wyvern the Talisman of Protection, for example.