Do you wonder that Draco Malfoy is based on Harry P. Flashman (started out in Tom Brown's Schools day?
1. Both of them are despicable aristocrats.
2. Even though both of them are villains, they are portrayed with positive side by another author (Fans for Draco, and Fraser for Flashman).
P.S- What do you view with more favor? Flashman or Flash Thompson.
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It wouldn't surprise me, Tom Brown's School Days is very much a precursor to Harry Potter -- although Rowling herself saved the Draco Malfoy character from full vilification to become a mere Butt Monkey.
#1 is because it's a common trope. Everybody knew that one snooty rich kid in high school, it's an easy way to express a character without much work. Which, given that Draco was mostly there to help define Harry and be an antagonist when the big bad wasn't appropriate, is about the right amount of characterization for plot importance.
#2 is the nature of fanfic. Given that the Potter series started out as stories Rowling told her own kids, I think it's incredibly unlikely that she developed the characters with an eye towards internet fandom would take to them. I don't see how the behaviors of third parties can be used as an argument that A is influenced by B.