See my sig for my take on it.
It originated in several short-lived threads speculating about future Pi features. Some of the contributors were ill-informed about both technology and the objectives of RPF. After several wild, extravagant suggestions were made, someone would request a Pink Pony and a Unicorn. The threads concerned would be locked soon after.
"If it ain't broke, fix it until it is."
“Raspberry Pi is a trademark of the Raspberry Pi Foundation”
The Pink Unicorn is a symbol of commitment to sensible discussion of future versions of the Raspberry Pi ™