Penises in the mammals

[TyRobbins]

I have heard that penises can be categorized into different types. One type is vascular. What are the other types and why are they classified as such?

[Noweia]

There are two main types: fibro-elastic and musculocavernous.

The musculocavernous penis of humans, horses, cats and dogs contains large cavernous spaces which fill up with blood during an erection.

The fibroelastic penis of the bull, ram, and boar contains more fibroelastic tissue with smaller cavernous spaces and a sigmoid (s-shaped) flexure.

[gen2]

There is also an os penis.  That is one that has a bone in it.  There is actually an AnswerBank article on this here

[rampart]

I'll never look at myself the same again.   ;) 

[Noweia]

The os penis isn't actually a type of penis - it is just a bone.  Some species (mainly those with musculocavernous types) have a bone in the penis and this is the os penis.  The most common example is the dog.