It is derived from ora, which is to shout or roar, and Nyan, which is a cats meow. It is the male version of a tsundere, where someone is at first confrontational and off putting, but slowly warms up to them. Just like how Tsundere is derived from tsun tsun (turn away in disgust) and dere dere (to show affection). It’s an analogy of a lion turning into a ...
That particular reference was to Daisuke Ono, who was the voice actor of the Sebastian butler in Black Butler. Daisuke plays himself in Lucky Star. There have been a crap-ton of Sebas-chans, though, which makes for a running gag.
In the 1974 anime Alps no Shoujo no Heidi the Butler is called Sebastian. I guess it all started there, but it might go back even further.
Anime featuring a butler called Sebastian (even though that might not be their real name):
1974, Alps no Shoujo no Heidi
1998, Princess Nine: Kisaragi Joshikou Yakyuubu
1999, To Heart
2005, Paradise Kiss
2005, Trinity ...
Body swapping has become a popular theme in Japanese media because it has been part of Japanese tradition and culture. Various Japanese beliefs has mentions of body swapping and this has become an inspiration for various literary forms in Japan which continued for centuries. This was evident when Makoto Shinkai was interviewed and asked for the inspiration ...