how to change the Category box on the Write Post screen

i have 6 categories and for each i have about 100 sub-categs (animals, birds, plants, insects… these are the categories and the sub-categs… you know… the animals, the birds…)

is there a way to change the Category box on the Write Post screen, so it shows just one category with its sub-categories only? having so many sub-categs is difficult to scroll to the one i want.

i want to change the write post page so i can chose what i want to write about, kind of…
[write about animals], [write about bids]…etc, and on click it opens a Write Post screen with the Category box containing just that category and its sub-categs.

thanx