IceBoss

Unidentified Life Form 51 is an unused creature that was originally intended to make an appearance in Metroid Prime. The bio-form loosely resembles a furless, blue-skinned primate, with its most distinguishing features being its six limbs (two pairs of arms and two legs), its tusks and the crystals which cover most, if not all, the creature's dorsum along with the back of its arms.

In its only existing animation (made by Derek Bonikowski), the creature exhibits behavior similar to that of a gorilla by standing upright and beating its chest with its first pair of arms while roaring before reverting back to its standing animation, in which it takes a partially flexed posture that seemingly allows its body weight to be pressed against the ground through the second pair of arms' knuckles. Its facial features, however, share greater similarities with those of a baboon's. The size of this creature is impossible to establish with precision, but its animation shows it slightly hunched and looking downwards at an angle, suggesting it was supposed to be looking down on Samus Aran, thus confirming it to be at least a few sizes taller than the heroine.

This would also hint that the animal was intended to be a boss or mini-boss.

Trivia

 * Its blue-colored skin, as well as its crystals which may be clusters of ice, suggest it was supposed to be fought in the winter climate of the Phendrana Drifts.
 * This creature would have possibly been one of the very few mammals to have made an appearance in the Metroid series of games. The only other notable mammals are Humans and Etecoons.
 * The unused creature's animation was originally found in Derek Bonikowski's website, in which it was shown alongside other animations featuring the beta Space Pirate, the Parasite Queen and Thardus. However, for reasons unknown, the ULF 51's video was removed.