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I dont have the books handy at the moment, but if a gobbo hero on a gigantic spider joins a unit of trolls will he stand out for targeting purposes? or will he be hidden?
i don't believe ridden monsters may join units.
in any other case, if he were riding a giant spider, he would be consider cavalry. trolls are monstrous infantry, so only the same troop type would get the 'look out sir'.