Nessus is an alien called Puppeteer featured in the novel: Ringworld

Heshe has two heads- but not two different personalities in each one, since they share the same brain stored in it's hump located on it's back.

Hisher gender is not determined because Puppeteers have three genders, and he never revealed his.

I keep forgetting that Puppeteers don't have tails, but just bear with me.

And yes- heshe is supposed to look like heshe only has three legs.

If you can't see this image well, just tell me and I'll retake the photo :)