I do see a lot of villains slowplaying when they flop monsters, even on wet boards. However, I think that he would probably raise with 2 pair + so I would discount those holdings. I don't like reraising since I think it's an overplay. However, I don't like folding either since we have some equity against his range:
Board: Th 8c 6c Ah
Dead:
equity win tie pots won pots tied
Hand 0: 53.800% 47.87% 05.93% 674 83.50 { AcTc, AsTs, As8s, As6s, 97s, AcTs, AsTc, Ac8d, Ac8h, Ac8s, As8d, As8h, Ac6d, Ac6h, Ac6s, As6d, As6h, 97o }
Hand 1: 46.200% 40.27% 05.93% 567 83.50 { AdTd }
His raising range is probably the same as what he is going to call it off with and from looking at the equity, we would need over 50% equity to make a reraise profitable. I would just call and evaluate. Most villains would not bet the river when checked to without the near nuts and you could fold if he were to bet OTR.
If he has demonstrated that he can slowplay OTF on a wet board with 2 pair+, then I would probably shove since his range would look more like this and we would have over 50% equity to make a shove profitable:
Board: Th 8c 6c Ah
Dead:
equity win tie pots won pots tied
Hand 0: 47.128% 43.68% 03.45% 1057 83.50 { AcAs, TcTs, 8d8h, 8d8s, 8h8s, 6d6h, 6d6s, 6h6s, AcTc, AsTs, As8s, As6s, Ts8s, 97s, 8d6d, 8h6h, 8s6s, AcTs, AsTc, Ac8d, Ac8h, Ac8s, As8d, As8h, Ac6d, Ac6h, Ac6s, As6d, As6h, Tc8d, Tc8h, Tc8s, Ts8d, Ts8h, 97o, 8d6h, 8d6s, 8h6d, 8h6s, 8s6d, 8s6h }
Hand 1: 52.872% 49.42% 03.45% 1196 83.50 { AdTd }