You can very conceivably see a loose player make a river bet here with A hi thinking (More like praying) that either you'll fold or you'll call and either split the pot with K's and 8's Ace kicker or "who knows maybe he'll [YOU] call for info or with a lower PKT pair than 8's (not including 7's) to either show his unluckiness or because he's dumb!" THAT LAST PART IS VERY UNLIKELY. But if you see enough hands you'll realize that it is very conceivable. And it's something to put into your equation. Even a player who is normally decent, but thinks you're awful might make this bet here assuming you'll fold. Or a player might make this bet with a worse hand because he/she wants to "fire three bullets!" because that's a noble thing to do and by golly if he gets called the player is just gonna be like "OK now I have a table image to work with!" As in he might be making a bluff he knows PROBABLY won't work but he rationalizes it in his own way.

With all that said I think you should NORMALLY BE FOLDING HERE. But there's a shitload of things to consider.

As for "Why should he put you on TPTK when you haven`t raised preflop? That doesn`t make sense. " if you've never seen an MP player limp with AK then you haven't played enough poker.Something being unusual is a porr reason to write it off as something that doesn't need to enter your mind.