So the Shuine's Horn is currently bugged. It gives toxic moves +25% damage, but does not convert them to water damage.
Furthermore, it also negates the resistance to toxic damage when the moves are used against crystal, earth, and digital temtem.
IE: toxic damage with shuine's horn vs Anatan does 125% damage, and against nessla does 250% damage. This is being exploited heavily in the competitive PvP scene since 1.5 and should really be fixed as it gives 1 temtem the ability to basically nuke entire teams.