Cinema 4D has a module called Hair which lets you add all sorts of hairy detail to your models. Here are the steps involved.

Start with your mouse
Here's the little guy posed and ready for some fur

Polygon selection
The model is made up of 4-sided shapes called polygons. The first step towards giving it fur is to select the polygons that need covering. Those selected polygons appear in red

Add the hair
Here's what the hair looks like when you apply it to the selected polygons. Although that's not really hair yet. Those yellow lines with the blue dots are just guides to show where the hair will go. It looks wrong doesn't it. For example it's all the same length and standing straight out.

Yikes!
This is how the fur would look if you let the computer render the fur with the guides set up like in step 3. See how the fur appears where the guides were. That's one hairy mouse. But now it's time to fix the fur

Hair cut
It's back to the hair guides. They've now been reduced to a length more suitable for a mouse. Notice that the hair is especially short around the nose. The hair is still standing straight out though.

Brushing the hair
I've now brushed the guides back to make the hair look like it's not standing straight out

Whiskers
The mouse needs some whiskers, but the whiskers need to be made of thicker hair than the fur, so they are done with their own settings. Here you can see how the whiskers' guides look after setting their length and brushing them

Adjusting the density
The mouse fur should be less dense at the edges of its growth, near the nose, feet and tail. In this mouse, the light areas allow full (dense) hair growth. Grey areas (like around the mouse's 'wrists') will allow less hair to grow, and the black areas won't let any hair to grow at all. Because that black, grey and white skin controls the hair density, it's called a 'density map'.

The final mouse
Now that the density, direction and length of the fur is set it's time to let the computer do its thing. When you set the computer to 'render' it produces a mouse like the one here, ready to put into the scene