It looks like your light is way to harsh. You will need to figure out a way to soften the light. With a bigger animal like the one pictured. You may want to think of other solutions away from the light box/tent thing. You can get off camera lighting that will work with your cameras built in flash.. A small speed light flash with an optical slave option/sensor. A 24 or 32 inch umbrella.. no not one for the rain.. And a light stand to hold it all. Id go with the bigger umbrella.. You can setup your nikon to trigger the flash using commander mode. Or you can spend the extra cash and buy a radio trigger and receiver.. trigger would go in the hotshoe on top the camera, the receiver would connect to the flash.. This wouldnt give you TTL ability with the flash, but it would allow you to at least shoot in manual.. Set a shutter speed to 1/200 ISO to 100, then a middle 5.6 to 8.0 aperture. Then the power on the flash you can raise and lower until you get the proper exposure.

My basic setup is a roll of white or black paper for my background, on a small background stand.. Then I use an Alienbee B800 light on a boom stand. That allows me to put the light directly over the top of my background. Then I attach a 24 inch beauty dish to the light. Then there is a diffusion sock that goes over the beauty dish to help soften the light. I then use a set of Paul C Buff Radio triggers to trigger the light. I use a Fujifilm Xpro2 and mostly the 18-55mm lens to shoot with. I shoot Raw, auto whitebalance. ISO 100, Aperture of 100, and a shutter speed of 1/125. I want to say the flash power is normally at 1/4 or 1/8.

With that setup I took this shot last night. My whitebalance is a bit off, I can see a bit of blue tint in the bottom of image on the white. But then again that could be my monitor.
BP-18-0022-4 by Shannon Donoho, on Flickr