I have the Arachnids on Normal Enemy Force mod, and the enemies from it will spawn properly as Arachnids. Rampant(Fixed)'s Arachnids however appear to just use the default biter graphics.

Thanks. I think in the next few updates I'll make it so that arachnids will always look like arachnids, not like biters.

https://mods.factorio.com/mod/Arachnids_enemy

Since the mod points to the old Arachnids mod which is deprecated, and this seems to be an update of them, maybe pointing the dependency to this instead would solve it?

I'll just move the animation from that mod into mine. Back when the "Arachnids" mod was being created, the author gave his permission. That's actually why this faction appeared in Rampant.

Would it be possible to have an option that would replace all default biter graphics with the Arachnids, also?