The usual worms that hatch in the environment are  Strongyloides stercoralis and Ancylostoma caninum. Both of these worms are really confusing in terms with their infective larvae. 

http://web.stanford.edu/class/humbio103/ParaSites2006/Strongylodiasis/morphology.html 

Dear Paweł,

In this case, it is important to consider that there are many species of nematode that are not parasites. Indeed, although almost 30,000 species have been described, of which about half are parasitic, the total number of nematode species has been estimated to be about 1 million. Anyway, whether the detail seen in the nematode in the third figure is a stylet, this suggests that the species is not parasite from the dog. All plant parasitic nematodes have a protrusible, hollow stylet, however not all stylet bearing nematodes are plant parasites. Thus, also consider the possibility of free-living nematodes: a world to be thoroughly studied!

Good luck in your investigation!

Best regards,

Vitor

Haven't you found any eggs in that sample? Most probably as the sample is mixed with grass some free-living nematodes may appear.

Hi, 

Based on all pictures, I can see an stylet which a structure typical of free-living or plant nematodes, not parasitic of animals.  Additionally, S. stercoralis and Ancylostoma spp.-larvae don't have this structure.  If you took the fecal sample from the soil, it's probable that it got contaminated with a free-living nematode.  I hope it helps!

I agree that they are most likely free-living nematodes contaminating the feces.  Good luck!  Again the case emphasizes how important it is to collect fresh fecal samples.

I agree with  Dr. Gardner, looks like Pratylenchus sp. MK

Dear pawel,  2,3,4-are plant parasites  While the remains are free living nematodes.   They r not larval stages of parasitic nematodes.