what does "natural special"mean? a specialist in nature?
All the way to school Grandma chattered away, as though it was perfectly normal for her to be walking Vinnie to school in the middle of the morning. She didn't even mention the fact that Vinnie had changed her clothes, though she must have noticed. The closer they got to school, the colder Vinnie became. She had seen a nature special once that told about people being frozen to death. They got sleepy, it said. The special must have been wrong. Vinnie wasn't the least bit sleepy. She was wide awake, so wide awake that her eyelids felt glued back. She was frozen and sweating at the same time. Her mouth and throat were so dry she couldn't have spit, much less spoken.

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The original text could have been written more clearly, for example: "She had seen a 'nature special' once on TV that told..". They are talking about a TV program. This (IMHO) is more of an 'Americanism'. We do hear this sometimes in England but in England we'd usually say it's a 'nature program'.

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Nature special most likely refers to a special documentary show. A documentary that is devoted to nature, something that might not be usual for the channel (like in a NatGeo channel or History channel).

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TV shows are called TV specials, normally when the usual schedule is interrupted (such as for extra-long episodes) or when the topic is of particular importance. In this case, special is short for TV special. A natural special is a TV special about nature.