せんとくん (Sento-kun) is the official mascot character for the event being held in Nara prefecture: "Celebrating the 1300th anniversary of Nara Heijo-kyo Capital in 2010." He has the appearance of a little boy with deer antlers growing out of his head.
Nara prefecture are famous for the deer that roam the parks, as well as it's many Buddhist temples.
(Children are called "Douji" in Buddhist art and are said to have sacred power.)
When his design was first revealed to the public, it received strong criticisms, with people calling him "creepy" and "a desecration of Buddhism." On the contrary, these criticisms ironically became publicity and made him famous.
As data was released of Sento-kun being depicted as a muscular, grown-up man using 3DCG modeling software called "MikuMikuDance," he was called "せんとさん (Sento-san)" in order to differentiate him from the more child-like Sento-kun.