In addition to boar hogs, dogs, cats, all primates and many other animals feature the mamma masculina.
I've been thinking about male mammal nipples ever since my anthro prof mentioned them in class last week.
Mammals all have one thing in common: Milk parts. That got me thinking about which male non-human mammals had visible nipples, and a google search led me to this article at Salon (incidentally, this was published on my birthday seven years ago).
A more thorough look into this is necessary, but for now I will rest easy knowing that my nipple search was not in vain. But don't bother looking for nipples on a platypus of either sex. Apparently the milk just kinds of leaks out of them....