A non-humanoid mount unit - eight legs, poisonous bite, spins webs... suffers from the same pwoblemErf-b1-p026 as the dwagons.
Spidews are heavy units.
Despite their Heavy nature, spidews may enter tunnels. This is how they were able to leave Gobwin Knob when it had no gate.
Alternate possibility: Spidews can scale walls.
Gobwin Knob - apparently their main source of non-flying mountsErf-b1-p001Erf-b1-p002Erf-b1-p123 and the common steeds of the Knights in Stanley's ServiceErf-b1-p111Erf-b1-p129.
Can also be used on their own, without a rider.Erf-b1-p025