The Ranger who pilots the Mastodon Dinozord in Mighty Morphin Power Rangers is the Black Ranger, so the answer is Black.
The original Black Ranger is Zack Taylor, one of the five teenagers recruited by Zordon to defend Angel Grove. His Mastodon Power Coin gives him access to the Mastodon Dinozord, a massive machine with a trunk, tusks, and other visual features inspired by the extinct elephant relative.
The Mastodon Dinozord is one of the five original Dinozords. When combined with the Tyrannosaurus, Triceratops, Sabertooth Tiger, and Pterodactyl Dinozords, it helps form the original Dino Megazord. In battle, its body contributes important structural sections to the combined robot.
The main source of confusion is that the team’s colors and pilots change across later eras. Adam Park later becomes the second Mighty Morphin Black Ranger, but the Mastodon association begins with Zack. The Yellow Ranger pilots the Sabertooth Tiger, while the Blue Ranger pilots the Triceratops.