The special mammal found only in the forests of Congo is the Okapi. Here are the details explaining why the correct answer is option 'D':

1. Introduction:
- The forests of Congo are known for their rich biodiversity, and one of the unique mammals found in these forests is the Okapi.

2. Description of the Okapi:
- The Okapi is a large mammal that resembles a cross between a giraffe and a zebra.
- It has a long neck like a giraffe, but its body is more similar to that of a horse.
- The Okapi has a reddish-brown coat with white stripes on its legs, resembling a zebra.
- It has a long tongue, which it uses to grasp leaves and shoots from trees.

3. Habitat:
- The Okapi is found exclusively in the dense rainforests of the Democratic Republic of Congo in Central Africa.
- These forests provide the Okapi with the perfect habitat, as they are rich in vegetation and provide ample cover.

4. Behavior and Diet:
- Okapis are primarily solitary animals and are most active during the day.
- They are herbivores, feeding on leaves, fruits, and other vegetation found in the forest.
- Because of their long tongue, they can easily strip leaves from branches.

5. Conservation Status:
- The Okapi is listed as endangered on the IUCN Red List of Threatened Species.
- The main threats to its survival are habitat loss due to deforestation and hunting for its meat and skin.
- Efforts are being made to protect the Okapi and its habitat, including the creation of protected areas and conservation programs.

6. Conclusion:
- The Okapi is a fascinating mammal found only in the forests of Congo.
- Its unique appearance and behavior make it a special and important species to conserve.
- By raising awareness and taking conservation measures, we can help ensure the survival of this remarkable animal.

In conclusion, the correct answer is option 'D' - Okapi, which is the special mammal found only in the forests of Congo.