Monsters freshly seeded into the outer forests after the ninth floor's playing field was expanded - the floor's designated grinding ground.
Cascadia mentions them in her sign-off: if the crawlers find themselves bored, the playing field's size was greatly expanded recently, and new mobs have been seeded in the outer reaches of the forests. 'So there's plenty of training to be had.'
Two useful facts sit inside that. The floor was enlarged recently - which fits her earlier remark that things have changed 'because of some recent developments' - and the enlargement was populated deliberately, giving crawlers somewhere to level that is not the war. For Carl specifically, monsters matter beyond experience: without them he has nothing to spend soul charge on, which is exactly the corner he is in inside the city, where Ceasefire rules prevent him from harming anything alive.
Undescribed; monsters placed in the outer reaches of the forests beyond the plains.
The floor's grinding population, and evidence that the ninth floor was expanded and restocked at short notice.