From Halopedia, the Halo wiki
Armored personnel carrier.
Mastodons are heavy troop carriers available to Commander Jerome. Each is armed with a M41 machine gun in a remotely operated weapon station, but the vehicle's primary offensive utility comes from the three units of infantry it can carry. Each of these units can engage the enemy using their weapons through the ABC's firing ports. The Mastodon can be upgraded to carry an additional infantry unit. If the Mastodon is destroyed the troops within will survive, but suffer heavy damage.
M650 Mastodons are widely used by the UNSC Army and colonial defense forces. Their large troop compartments enable them to carry heavy infantry (such as Spartans in powered assault armor) and their armor protects against most heavy weapons, but their size made them difficult to deploy and they lack the mobility of smaller (but far less protected) Warthog troop carriers. With troops in short supply on the Ark—and reinforcement unlikely—Jerome has investigated starting production of these vehicles to minimize infantry casualties.