Ganesh is a popular deity of Shingon Sect, known as Kankiten 歓喜天 http://www.onmarkproductions.com/html/kankiten-idaten-other-tenbu.html.
In fact, there is a lot of similarity between Japanese deities and
Asia deities. If you look at history, Buddhism travel from India,
into China then Japan. Of course, when the state embrace the
religion automatically a lot of patron deities become popular with
the Jap.
In fact, there is another deity known
in Asia as 玄武上帝 http://www.onmarkproductions.com/html/28-moon-stations.html.
He was adopted as a patron bodhisattva of Nichiren Buddhist
sect. He is known as 北斗妙見菩薩 due to his appearance to Nichiren in
time of danger.
Most of
us will classified the deities based on geographic and
culture. However, the deities is able to manifest themselves
in certain manner to teach and guide the people which is beyond our
human comprehension. Perhaps this is Lotus Sutra implication
Skillful Mean,方便行.
@ Reborn76
Haha nice info... Another inter-related deity.. I was thinking
whether is it the deity stepping on serpent and tortoise... indeed
it is him lol (googled his picture lol)
@Wiser
Like I said... devotees will often linked up several roles for
their favourite deities... For Lord Ganesan, or Remover of
Obstacles. His title is sooo wide... It's like, students can pray
for good results, couples for good marriage life, workers for
good working life, good bosses, etc this list goes on...
But I am not very comfortable with the idea that Guanyin was
supposed to "married" to Lord Ganesan... lol
Anyway... always remember... 皈依佛，永不皈依天魔外道 ok...? 
