Are collisions with animals something to be worried about when cycling in the country? I know that driving at dusk in wooded or otherwise open areas greatly increses your risk of collision with an animal, whether that is a deer or other wildlife. I would like to know if when cycling at speed this is something I should keep an eye out for. Have there been instances where a cyclist on a road has collided with a large animal?
Sure, there have been cases of hitting an animal in the day or night. Here is an article of a cyclist being attacked by a moose, for example.  In addition, you need to worry about road hazards since you won't see them as early. 

Obviously, the amount of hazard depends on the particular location, but you should have good lighting and reflective gear at night as well as be prepared to slow down and stop if necessary. If the area is known to have hazards, it may be advisable to go a little slower so you can stop quicker if needed.