Bobby Hill from King of the Hill is actually Bill Dautrive's kid. If you watched King of the Hill, you'd know that Hank Hill has a narrow urethra, which makes it harder to have children. Their only child with Peggy, Hank's wife, is Bobby.
Bill Dautrive is one of Hank's neighbors and high school friends. Bill used to be a star football player, now he's fat, alone, divorced and depressed most of the time. And he has a crush on Hank's wife.
Bill flirts with Peggy a lot, but Peggy doesn't like it. The theory is that Peggy had an intimate relationship with Bill at one point before Bobby was born. If you look at Bobby, he shares some traits with Bill. He's fat, he's socially awkward, he even looks like him a bit.
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This
link explains it a bit more than I could.