Rocktite

Rocktites are Gohma-like monsters that dwell deep within caves. These creatures lay dormant atop the ceilings of caves, waiting for trains to pass by to attack. When awakened, Rocktites chase and attempt to tackle the intruders.

Strategy
In order to protect the Spirit Train, Link must use his Cannon to fire Bombs at the monster's eye in order to slow it down and eventually defeat it. If Link is not fast enough, the monster will lunge at the train and crush it. After a certain number of hits to the eye, it will be defeated.

A certain Rocktite within the Sand Realm is the first of Three Trials to guard the passageway to the Sand Temple. Its behavior is similar to the first encountered Rocktite, but it sometimes keeps its eye shut, leaving its only vulnerable point protected. Link must fire at explosive barrels embedded into the cave walls in order to harm the creature and open its eye.

The third and final Rocktite can be found in a tunnel leading from the Fire Realm to the Sand Realm. Fighting this one is optional and it is only purpose is to stop Link from delivering Dark Ore to Linebeck III. The Rocktite is fought similar to the one in the Sand Realm, but will also send out Baby Rocktites when it flees into the shadows of the cave.

Trivia

 * Though not directly stated, the Rocktite may be related to the Tektite as they both share the characteristics of a single eye and four legs as well as both having the "-tite" suffix in their name. Zelda hints this if Link makes it past the Rocktite in the Fire Realm by stating, "What if all those little ones grew into something like that? ...It's too gross to even think about!".
 * In an early trailer of, Link is seen being pursued inside a cave by a giant arachnid with an eye on its front, similar to the Rocktite.