Is it just me? Here are the pictures:
Infernape and Emboar's Body Art looks similar because Both Infernape and Emboar are Inspired from the Character in the Book '' The Journey to The West". Infernape is Inspired by Sun Wukong, The Monkey King In The Book'' Emboar is Inspired by Zhu Wuneng, The Demon Pig in The Book "The Journey To The West".
There's not really a reason (other than Game Freak getting lazy), and probably wasn't intended to look that way, but they are sort of similar; you could say that. The colors are certainly similar, so I can see why you'd think that. Do keep in mind though this happens a lot; back when we didn't know much about Unova's new Pokemon, many of us believed that Alomomola was an evolution on the pathetic Luvdisc.
Only game freak knows why. They made it like that..... maybe they are cousins. Write a letter to game freak or something... I doubt they will reply.