Rocktite

Rocktites are Gohma-like monsters that dwell deep within caves. The 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 his train, Link must use his cannon to fire bombs at the monster's eye in order to slow it down and eventually kill 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 eventually die.

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's only purpose is to stop Link from delivering Dark Ore to Linebeck III. This 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

 * In a beta Trailer to Twilight Princess, Link is seen being pursued by a giant, gray colored arachnid with an eye on its front, while inside a cave, theorized by many to be an early adaption of Armogohma.
 * 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!".
 * The optional Rocktite's offspring (that are on-screen) can all be killed by exploding a barrel.
 * Link can get a slight head start by firing a bomb at the rocktite right before the cutscene where it drops from the ceiling plays. If done correctly, the Rocktite will run into the floating bomb and take damage.
 * The Rocktite has its own theme song, which will change as it gets closer to the train.
 * The part of the song that plays during the Rocktite battles when it is far away is similar to the Goron Mines song from Twilight Princess.