As though they are peculiar. When they're just regular humans who happen not to have a sibling/s.
Since I joined MN when DD (now 18) was a toddler there have been numerous threads asking if onlies are lonely, selfish or bored. More difficult to parent, to teach to share, to socialise.
I've never come across a thread asking what it's like to be the 3rd child in a family of 4. It was fine in case anyone's wondering.