Unlike Atavisms, which are instinctual representations of the Horror, Nightmares have to be learned by the Begotten, either by traveling the Primordial Dream, or by being a witness to another Beast, or even another supernatural. Certain Families are drawn instinctively toward specific Nightmares, but the fears called up by these powers are deep and fundamental enough that any Beast can manifest any Nightmare with equal facility. Familial differences are more apparent in the hallucinations and sense-images the victims suffer.
Nightmares are expressions of deep-buried fears that go deeper even than the subconscious mind, straight to the collective soul of humanity. These fears are not always debiliating - someone in the grip of terror runs faster than an unfrightened person. While Nightmares are “all in your head” in the sense that only the victim experiences the effects and sees the hallucinations, that’s not the same thing as Nightmares being “illusionary” or somehow “not real.” The effects of Nightmares are very real and are quite capable of inflicting somatic injury or even death.