Good ol’ post hoc ergo propter hoc!

OK so had to Google that. Essentially means Event A happened and then Event B happened therefore Event A caused Event B. For example the rooster clucks in the morning therefore the rooster causes morning or whatever.

Actually, fatphobia the whole relies on a giant post hoc fallacy: “I met a fat person who engages in behavior I consider immoral; therefore, behavior I consider immoral must make people fat.”

OK I'm going to assume this chick means the overconsumption of calories by immoral behaviour. Overconsumption isn't usually conflated with immorality. See here's the thing: It happens with 100% rate that people gain weight through the overconsumption of calories and when you take away the overconsumption through reducing calories or eating to your TDEE you lose weight or stay the same weight. That's more than a correlation. In my earlier rooster example if you kill the rooster the sun still comes up. We also have an understanding of the biological processes that dictate weight gain.

fallacy of unwarranted extrapolation (“I met a fat person who engages in behavior I consider immoral; therefore, all fat people engage in behavior I consider immoral”)

It's not just because I saw one fat person over consume that i formed this opinion.

the just world fallacy (“Fat people are treated so terribly, the benevolent cosmos would not allow people to be fat if they weren’t engaging in immoral behavior that merited terrible treatment”),

Define "terrible treatment." Because I'm sorry but things like paying for more than one airplane seat fat people deserve that because they're using more than one seat. But no I along with most other people don't want anything genuinely bad to happen to fat people.

one big fallacious gestalt with hellfire belching from its skeletal eye sockets.

You don't have to talk like that. It doesn't make you sound sophisticated and clever; it makes you sound like a wanker.

Also strawman fallacy.