Why is there a PostgreSQL Turtle?
As stated in the official Identity Guidelines


  In Japan, a turtle logo is used in place of the elephant due to domestic terrorist symbolism.


The elephant has negative connotations in Japan because members of a cult which uses an elephant as its logo were responsible for some terrorist attacks.

The group in question wore blue elephant masks while campaigning for Japan's parliament a few years before their murder spree, rather than it being exactly a logo. Personally, I would rather not post links to pages about their exploits, but if you want to go looking it doesn't take two minutes on a google search to find articles if you use words like: elephant deadly sarin gas tokyo

You can see the turtle iconography used quite extensively on the Japanese Language PostgreSQL site

While other Japanese companies like Zojirushi may retain use of the Blue Elephant iconography, that company in particular dates back to 1918 and its branding dates from the 60's. PostgresSQL's use of elephant iconography dates from only the late 90's which corresponds very nearly with the horrible events previously mentioned and would have no doubt been a contributing factor in the community decision to not use the imagery in Japan.