This is a list of cross-platform portable Open Source databases.

XBase
https://sourceforge.net/projects/xdb/
DBase compatible database.

QDBM
https://dbmx.net/qdbm/
Works on several platforms. GNU gdbm compatible replacement.

SQLite
https://www.sqlite.org/index.html
Public domain SQL database by the developer of FOSSIL SCM.

PostgreSQL
https://www.postgresql.org/
High capacity SQL database.