Arial Rounded didn't have its own wikipedia page, only a category named in the page for Arial so the information below is from said page. Even then there wasn't much info on Arial Rounded. However it appeared that rounded is one of the more recent Arial family things. Not too familiar with typography at the moment...
Arial Rounded typeface (Arial)
Who created it?
10-person team, led by Robin Nicholas and Patricia Saunders
When was it made?
Designed in 1982
In 2005, Robin Nicholas said "It was designed as a generic sans serif; almost a bland sans serif." (Arial)
Arial is similar to Helvetica