This word is acceptable for play in the US & UK dictionaries that are being used in the following games:

• Lexulous
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n. Large or relatively large animals, as of a particular region or period, considered as a group.

n. The large animals of a given region or time, considered as a group.

n. A treatise on such a group of large animals.

"In a future where all megafauna is extinct and where technologically advanced humans are highly skilled at genetic engineering, future people 500 years hence genetically create ungulate-like grassland people, cold-weather tundra people, scansorial forest and woodland people, and gilled, seal-like aquatic people (Dixon 1990)."

Wikipedia article on megafauna   

megafaunal