Slitherine

The army list says they are armed with arquebus if brought out of the core troop area (0 - 4) but with muskets if selected out of the option troop group.

Is this an error? I would have thought by the 30yrs war they would have all been musket.

I think its actually two different troops. The ones you refer to are the Forlorn Hope of Carabins and can be either LH or dragoons

The other option is actually called Dragoons.
Did not see exactly what the Forlorn Hope were all about but hope that solves your dilema.

I am not aware of any other troops in this period still using arquebus. It looks more like a typo to me, the French army for the previous period has exactly the same Forlorn hope troops.

Why would one group of dragoons use muskets while another use weapons from an earlier period?

I would think this is correct.

The Carabins were a light horse unit armed with carbines who occasionally dismounted to fight in a manner akin to dragoons (i.e. move mounted but fight on foot) and thus when they fight as dragoons they are still armed with the carbine, which for foot troops is treated as an arquebus.

The dragoons later in the list ARE dragoons armed with musket!

Simples!

Don