Interesting question, Sleipnir approves. The short answer is, it sort of depends on your creature's body design.
From the reading it sounds like you are describing something that would function similarly to a centaur...possibly. It's also possible that the creature would travel on all six legs with the front pair functioning similar to a gorilla where they can be used for both travel and more fine motor skills.
The finer you want the motor skills to be the less likely your creature will use the front pair of limbs for locomotion.
What your creature is specialized for could range pretty widely as well.
I can envision a prairie/grassland creature with bison or rhino like characteristics using its size and bulk as a defense...maybe some horns just for fun.
- This creature would be low to the ground with strong legs. It would be very stable.
- The complications of extra legs in the creature's gait could be removed/mitigated by short stout legs and a slightly longer body than traditional prairie creatures
- This body design doesn't work particularly great for giving it precision digits, if you want to do that you probably need something more akin to a centaur body design with an upright portion that doesn't touch the ground.
- It likely wouldn't be all that fast but you can probably make an argument that it could move pretty quick in a strait line.
You could also have a tree-dwelling, monkey like creature, I can see an extra set of limbs being great for climbing.
- All six limbs would need to have gripping digits. Like monkeys these don't have to have opposable thumbs, the can simply hook to latch onto tree branches.
- If you are going to have thumbs it makes the most sense to develop them on the middle set of arms. Using the top and bottom set for climbing would be the most stable, then the middle arms could manipulate whatever while climbing.
- I see no inherent difficulty with gait in this setup.
Aquatic creatures also make a lot of sense.
- Extra fins could make the creature faster and more agile in the water.
- If you want an aquatic creature with actual hands you probably need the extra set of arms to make them remain decent swimmers.
- I find this creature less likely than the others to develop digits as survival, especially earlier in their evolution is going to depend on speed and agility in the water...not much need to evolve from that.
I struggle to see extra legs being beneficial in the mountains...but it could probably work, life finds a way to evolve in bizarre and unexpected ways all the time. Nothing immediately comes to mind on this one...the extra legs would be a hazard for a mountain goat as an example.
Predators are interesting with this setup...but complicated. I am immediately reminded of this guy from Avatar super awesome looking, and frankly a bit terrifying. Its actually what I was picturing though probably smaller in my sonic hunter question.
The weird thing about predators...rarely if ever are they the biggest creature.
- In Africa you have lions and cheetahs, they prey on animals much larger than themselves
- In North America you have wolves and cougars/bobcats which are quite a bit smaller than most of their prey
- Bears may be a decent example...but grizzly bears primarily go fishing, they don't really hunt deer or anything, they are not equipped to chase things for long
- In jungles you have a jaguar, they tend to hunt sloths, tapirs and monkeys. So this may be the best example of a predator that goes after smaller prey.
On a creature that is generally on the ground with a small frame, extra legs seem like they could be more a problem than a benefit...
- I suppose if you altered the frame a bit you could make it work. If you give the creature four front legs and two back legs I think you could get a gait that works. Plus the extra front legs would allow the predator to grip better when they latch onto their prey.
- When you specialize your digits to help you grab on and tear open something else it's not super likely you are going to evolve human manual dexterity.
- Predators are high energy creatures which makes size a liability, being bigger requires consuming more food more often, in lean years, be it from overhunting or a drought...that's not great, so a cool as the six-legged battlecat from avatar is...it doesn't seem real practical.
- It'd be strange but I can picture six legs where while hunting the middle legs are basically wrapped around the creature's abdomen but it looks creepy and weird in my head.
- As in avatar I can see six legged creatures working better in a world of larger creatures.