Uh, wait no nevermind what I said below for some reason I was sure that this was my mod about my Giant Norscan Spiders
Try the following keys:

zerg_grn_frost_giant_spiders for Frost Giant Spiders
zerg_grn_frost_arachnarok_spider_01 for Frost Arachnarok
zerg_grn_frost_arachnarok_spider_02_ror for Burning Frost Arachnarok
zerg_ksl_frost_giant_spiders for Frost Giant Spiders (Ostankya)
zerg_ksl_frost_arachnarok_spider_01 for Frost Arachnarok (Ostankya)
zerg_ksl_frost_arachnarok_spider_02_ror for Burning Frost Arachnarok (Ostankya)

I never used the console to spawn stuff before so I'll just take a wild guess and assume that it's the unit main key, try this one: zerg_nor_norscan_giant_spiders

What’s the unit codes if I wanted to use the console to spawn them?

Ok. I had meant his monster pens like you assumed. Thank you for replying!

Unless you are suggesting to forcibly slap the first two units inside Rakarth's horde beasts building chain (the one that gives you units like the War Hydra and the Black Dragon), no I can't ATM.
I still have no clue about how the Monster Pens mechanic works and don't know how to properly add custom units inside that particular system.

Could you give this unit to Rakarth?

Possibly I guess (I do struggle with English at times), but I was undecided between the two and went the other way.
Would be kinda weird to change the unit's name now so, unless we are talking about an atrocious grammatical error filled with Chaos energy, I would be inclined to let it slide.

Shouldn't it be "Giant Frost Spiders" instead of "Frost Giant Spiders"?

Cool man!

Yeah, riderless Arachnaroks would work.

Another outstanding job, my friend, I love the new unit cards!

How about a small compromise? Give Frost Giant Spiders to Ostankya and possible to create a non-rider Frost Arachnarok for her faction?

That's why I always try to go out of my way to point out that my opinions are only that! Yup, Kislev is ice and Ostankya is haunted forest was my entire thought process. I just like to overemphasize thematic uniqueness and figured I'd share the thought because you never know! I knew it was going to be more complex than I was making it out to be, about the entire extent of my knowledge on the matter is that Dilaguna seems to use orcs as a base for many of their units.
Thank you for all the work you do regardless!

I respect your opinion but I don't exactly understand the whole "this doesn't fit Greenskins" thing when we are literally talking about reskinned Greenskins' units.
It's not like Kislev have some kind of exclusivity deal with everything related to winter and ice you know?
Anyway regarding your question: unless there are vanilla's animation sets with Kislev riders on a Arachnarok ready to be used, I can't do anything about that since I don't make custom animations and I'm not sure that I could achieve that even with a custom tech skeleton since I'm still reading about it and I don't know its limits yet.
BTW I can't simply "reskin" the lil goblins into full Kislev soldiers, animations and models' skeletons don't work like that.

You know... Personal opinion, I don't feel this fits the majority of greenskin factions. HOWEVER, this would actually be pretty hype on Ostankya! Now I'm trying to think of something theme-appropriate to replace the goblins on the arachnaroks with... I've never paid attention to the height difference between a goblin and a human but I wonder if those crossbowmen from the Ostankya DLC would work. They'd probably have to be bowmen of some sort though, to make reskinning the goblins work. Kislev archers or the horse archer guys? Hmmm...

Skyrim flashbacks

SPIDERS

Merci