Why did Riddick save the pup?

There is nothing really in the movie that specifically states why he saved the pup, but I will share my thoughts on this.

I think b/c he has a soft spot for animals as we have seen in other Riddick movies that he decides to care for the animal as he is alone on the planet and it would make a good companion.

But, that being said, I don't think Riddick does anything by chance, I also think he had a plan for using the pup for his survival now and in the future. As having a rather big wolf like animal around on a hostile planet could come in handy.

As we all know ,Riddick had to get through the canyon where the Mud Demons resided. By the time he had found the pup, he knew what he had to do. After seeing what the Mud Demons can do, specifically poison anything with their bite, he figured that if he was to use small doses of the poison on himself so he could become resistant just in case he was bitten when trying to cross the pool with the Mud Demons.

At this time he finds the pup and I think he decides to first test the poison on the pup and if the pup did not die quickly he knew he could inject the poison into himself with confidence that he too will be able to survive.

This was addressed in an interview with the film's Writer/Director David Twohy. Riddick initially saved the dog for purely selfish reasons but ultimately came to like and respect it as a companion.

This was a little odd because the hero baddie creature, we call them Mud Demons in our production circles. Nobody calls them that in the movie. Those were actually designed by Patrick Tatopoulos, who was my creature designer on Pitch Black and who is now a production designer of note.

And then when it came to jackals, Patrick gave us some early sketches, and those were refined by other artists, because a pet jackal in the story that he finds is a puppy. At first he was using it as a tool to help inoculate himself against these Mud Demons. But then the puppy grows on him and grows literally and figuratively on him. And it becomes his companion on this world. So, at that point it went through a series of design evolutions, including with the creature house up in Montreal who did the CG work. But yeah, you are still building some fabricated pieces, not animatronics, that try to emulate life by blinking. But if Riddick has to, in a close-up, touch the animal's hide and run his fingers through it, then certainly that will be a build piece you can physically touch. But anytime it's the full grown jackal, or you need a face along with Riddick running and jumping, forget it. It's CG now

_{^{FirstShowing: Interview: 'Riddick' Writer/Director David Twohy on Budgets & Sci-Fi}}