As you continue your adventures through the world and checking off the different story quests, you will eventually come across a quest where you will need to find Rathian tracks.
These Rathian tracks will be marked as ??? on the floor, when you find them they will add to your total amount of needed tracks that you must find.
Please note though that the locations can actually be rather random. However, the tracks do indeed spawn in the areas found in the guide below.
What actually happens is that if the tracks are not where they should be for you then chances are they are not going to be in that area at all.
Thus to solve this you will need to leave the area completely, then return until they do spawn where they should be. Keep doing this until you have enough to progress the story.
Related Guides / Links: