Teachable agent is a type of pedagogical agent which instantiates Learning-by-Teaching theory through simulating a "naive" learner in order to motivate students to teach it. This paper discusses the limitation of existing teachable agents and incorporates intrinsic motivation to the agent model to enable teachable agents with initiative behaviors in fulfilling three perspectives of educational requirements, namely Teachability, Practicability, and Affectivability. The proposed Motivated Teachable Agent is realized by a goal-oriented approach, and through executing the goal hierarchy, the teachable agent can follow its intrinsic motivation to initiatively interact with students and arouse their learning interest. A use case study illustrates the effectiveness of this approach. (Contains 3 tables and 9 figures.)