I think a raw umber or burnt sienna oil paint wash would help bring out the surface texture. Photos of rhinos and elephants may give you some ideas about where dust and dirt would accumulate. If you're planning on putting it on a ground base you could match the dirt on the feet. These animals, like elephants have a wide foot print so they would not really sink into the ground unless they've been in muddy areas. You might try dried mud on the ankles with darker, wetter looking mud on and between the toes.
BTW, it looks great. Which kit is this and which paints did you use?