Can't it be both?

[He snickers. Now he's partially tempted to try that... what hell have you wrought, Twilight?]

Anyway, Toxicroak's not just Poison/Fighting. With the right ability, they're immune to water. It heals them up, even if they do gain a fire weakness in exchange.

[And sun... but eh, there's always some kind of tradeoff when it comes to tough Pokémon.]