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In the game there is a magnum-class pistol called "Horse" which has the description "Perfect Pistol for a mercenary on a bizarre adventure."

This is a reference to the manga series JoJo's Bizarre Adventure, specifically about character Hol Horse's gun "The Emperor", which is a revolver combined with an automatic pistol. Not only are the design and names similar, but the description also hints to the JoJo series as well.
Contributed by ZpaceJ0ck0
Upon rescuing Ironside, he will have a wooden sword for sale. Its description states "It's dangerous to go alone. You should take this.", which is a reference to the first Legend of Zelda game. There is also a buster sword hidden in the sword pile behind his tent, a reference to Final Fantasy VII.
Contributed by Tuli0hWut
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The Brass Snail is a reference to the Metal Slug series, a major source of inspiration for Mercenary Kings.
Contributed by G-Haven