She dreams of weight.
Something heavier than she is, heavier than she can bear, is pushing her down. Crushing her. She struggles to stay on her feet, to stagger uphill against the weight. Nopony else is affected, just her.
She grows stiff and rigid, forcing the weight back at the cost of being able to move. Every step is torture, grinding, creaking, when she longs to run and play. Inside her a fire burns, and it burns hotter and hotter with each moment, but she cannot move. She cannot release it.
In some of the dreams, she twists, and thrashes, and the rigidity cracks, the weight releases, and for moments, she knows freedom. And then the whole world burns from the fire inside her.
She is carrying something up the hill, carrying something unbelievably heavy, her body stiff and barely able to move so it can be strong enough to carry the weight. Sometimes the weight slips, and she watches a ball of fire roll down the mountain, setting fire to everything in its path. Then the world turns into a flare of white, and she wakes with tears on her face, feeling like a failure.
Sometimes she lifts the weight up with her telekinesis. Up, up, up, so high that she almost feels free of it, but it is always pressing on her horn, always grinding her down. She can take the fire out of herself, she can lift the weight, but it's always there, pressing on her, and if she ever drops it, the world will burn.
Sometimes the weight is a crown. Not the one she wears, the simple circlet that marks her as the heir, but the one her father wears, golden and heavy. It's on her head, and she can barely stand up, but if she shakes it off something terrible will happen.
The dreams about manticores and dragons come as a relief. She can just be afraid for her life then, or Luna's. She doesn't have to fear that the whole world will burn if her strength ever falters under the weight.