From PostgreSQL wiki
The PostgreSQL logo is an elephant, known as "Slonik". Multiple versions of the logo are available.
Use of the logo is subject to our Trademark Policy.
1 color blue
1 color white
The logo is also available as a favicon : http://wiki.postgresql.org/favicon.ico
In the email thread postgreSQL and history of relational databases, Oleg Bartunov states that the logo idea probably originated from a PostgreSQL logo thread on the pgsql-hackers mailing list. Quote:
[...] but if you want an animal-based logo, how about some sort of elephant? After all, as the Agatha Christie title read, elephants can remember ...