I’ve done a little of this, though my situation is somewhat different from yours since I live in the wide-open southwest and am usually photographing animals against the night sky. Even on a moonless night the sky is always brighter than animals in the foreground. If you are working in forest your animals and your backgrounds may be the same brightness, which I think complicates things a bit. If you use a long exposure for the background, (as I have on the examples below), you’ll end up illuminating the foreground and getting double exposure where the animal is, since it will undoubtedly move during the long exposure. Your best bet may well be light painting or multiple flashes to illuminate different parts of the scene.
I like the effect you got on the shot you posted by the way.
Chihuahuan Hook-nosed Snake, New Mexico. by Bill Gorum on 500px.com
This picture is a single exposure, with flash illuminating the snake and the shutter left open for 30 seconds to pick up the lights from scattered houses and the sky. The orbs in the sky are out of focus stars. One thing I dislike in pictures taken this way is the black middle ground. Basically, the middle ground is too far away to be illuminated by the flash, but it’s also far darker than the sky, so gets no appreciable exposure during the 30 second that the shutter is open.
Sonoran Gopher Snake, Pituophis catenifer affinis by Bill Gorum on 500px.com
Here’s one where two exposures were combined, though strictly speaking, this could have been done with a single exposure too. In this case most of the middle ground has been eliminated because the terrain was very flat, and the vegetation right at the edge of the road was close enough to receive light from the flash.
Couch Spadefoot, (Scaphiopus couchii). by Bill Gorum on 500px.com
This is another single exposure. 30 seconds to get the stormy night sky, with two flashes, one aimed at the spadefoot and the other aimed at the vegetation on the opposite shoulder of the road. All the orbs in the upper left are out of focus rain drops illuminated by the flash.
Western Diamond-backed Rattlesnake ambush coil. by Bill Gorum on 500px.com
This one was taken before it was completely dark. In this case I used HDR to squeeze the contrast range all into one picture. This is all available light, no flash at all. 5 frames, bracketed two stops apart, (10 stops total range).
Great Plains Toad, (Anaxyrus cognatus), and full moon. by Bill Gorum on 500px.com
Here I just wanted the toad and the full moon. Single exposure, one flash on the toad, one on the grass behind it, 30 seconds. If I wanted the moon in focus, I could have done this with two exposures, one focused on the toad, the other on the moon. The moon would have been smaller then, though.
Prairie Rattlesnake on shoulder of highway in New Mexico. by Bill Gorum on 500px.com
This one’s a combination of at least three different exposures. I needed a different exposure for the sky, the lights, and the snake. The camera was sitting on the ground and I had to be very careful not to move it as I changed exposures.
Prairie Rattlesnake and Milky Way. by Bill Gorum on 500px.com
This is a combo of two exposures. The snake was shot with a low ISO, small aperture, and flash. The starry sky needed a high ISO, lens wide-open, and a long exposure. I did forgot to re-focus for the stars exposure, so the stars are out of focus.
Same process as above, but I remembered to re-focus.