Social Bunny

The Sims 2
The Social Bunny is an NPC that is the sims' imaginary friend. He comes when Sims have a low social need. They come in three colors which are blue, yellow, and pink and only the lonely Sim can see them. Just having the Social Bunny around raises the social need, but the lonely Sim can interact with him anyway to raise it, ranging from friendly kisses to outright fighting. The Social Bunny leaves when the bar is back up to the halfway point.

If multiple Sims are in social failure, the other bunnies will be seen when a lonely Sim is active. However, they will be transparent, and cannot be interacted with. The bunnies may interact with each other, though. Generally, ones of a different color will be friendly with each other, with blue and pink ones going romantic. However, much different sparks fly when two bunnies of the same color meet, as they'll actually fight with each other.

There has also been a bug where the social bunny can actually become a friend. (eg. He come to parties and he can visit and possible all the other things that normal friends can do)

The fact the Social Bunny is only seen by the active Sim is a reference to the films Harvey and Donnie Darko in which the main character can see a 6 foot tall rabbit who is invisible to everyone else.

The Sims 3
In the Sims 3 the social bunny only appears in a cameo in a poster under decor, where it seems he has terrible dance moves.