Step 1: Understand Proboscidea.
The order Proboscidea includes all elephant-like animals, including both modern elephants and their extinct relatives.
Step 2: Identify the Proboscidea member.
- Hipparion is an extinct genus of horse-like mammals, not a Proboscidea.
- Mastodon is a member of the Proboscidea family, related to elephants but distinct in its size and shape.
- Indratherium is an extinct genus of mammals but not a member of the Proboscidea family.
- Sivapithecus is an extinct genus of primates, unrelated to the Proboscidea family.
Thus, the correct answer is Mastodon.
Final Answer:
\[
\boxed{\text{Mastodon}}
\]