1.1 North American A quadrilateral with no sides parallel.
1.2 British A quadrilateral with one pair of sides parallel.
Source: Oxford Dictionaries
There doesn't seem to be a non-mathematical definition for this word. For that reason (and since the definition pertains to geometry), I'm certain trapezium is simply describing the physical shape of India. By looking at a map you can see how the shape matches one of the definitions above.
As you can see, the shape isn't perfect, but if you approximate, it definitely comes out as a trapezium by either the North American or British definition.
Since it describes India itself, the quote is saying zoonotic ogres live within the trapezium-shaped India.