1. So, what is this type all about?
The idea behind this type is to partition Bug-type into more than one group Main idea why this type exists is because of the Spider sub-archetype, as well as various Scorpion-based species that are not in Bug-type but in Bug Egg Group.
2.Where does this name come from?
The name comes from the actual animal group Arachnids.
3. Does this type have an in-game source?
Not really. Spider archetype is slowly becoming a sub-archetype, but other forms of Arachnids are not even a coherent sub-group. Currently, only relevant move is Spider Web.
4. What would be the ideas behind such a type?
Various forms of Arachnids and Arachnomorpha could be in this type. It would largely be a deviation of Bug-type though, perhaps a slightly upgraded form.
5. What would be the type interactions of such a type?
Mostly same as Bug-type, but without the Flying super-effectiveness and Grass effectiveness.
6. What would be the distribution of such a type?
Largely limited to mid-game Pokémon, as most Spiders and Scorpions in Pokémon are mid-game species besides Spinarak.
7. Future comments:
More kaiju-style arachnids, like the few seen in Monster Hunter can be added.