Autonomous Database is able to connect Non-Oracle Database (DB2, Teradata, SQL Server etc) via Oracle Database Gateway.
Oracle Database Gateway supports heterogeneous environments and eliminates the need to customize your applications to access data from non-Oracle systems. Autonomous Database supports creating database links only if the target gateway is accessible through a public IP or public hostname, or using an Oracle Database Gateway. Oracle Database Gateway provide queries and applications with access to data anywhere in a distributed database system without knowing either the location of the data or how it is stored.
Use DBMS_CLOUD_ADMIN.CREATE_DATABASE_LINK to create database links from an Autonomous Database to an Oracle Database Gateway to access Non-Oracle databases.
For more information on Oracle Database Gateway, see Oracle Database Gateways