The day Monster kind was sealed into Mt. Ebott, the mages added a little extra insurance. A curse.
Human SOULS may pass. But Human forms may not.
Young Frisk must navigate the dangers under the mountain, a little less human than before...
The following monster-sonas:
Shrimp dog belongs to Clickclackmoomoo
Galaxy (bat monster) belongs to Galaxy Story
Pointed-eared monster belongs to @geist19