Scary Monster

Scary Monster (スケアリー・モンスター) is an ability featured in Part VII: Steel Ball Run, seen as part of the stand, Scary Monsters (plural). It is named after the David Bowie album Scary Monsters (and Super Creeps). 

Appearance/Personality
While Scary Monster's acts like a stand, it is debatable on whether or not it is considered a "true" stand and not just the lingering effect of the stand Scary Monsters permenantly fusing to Diego after obtaining the Turbo Eye. As the stand is bound to Diego's body, it's appearance is based on the form Dio takes, but his tail always retains the words 'Dio' repeatably written on it.

Powers
Morph: Diego is capable of changing his body from that of a human to that of a half-dinosaur, fully fledged or partial form of it. While in his morph status he has an increased stamina, strength, speed and vision along with being capable of eating anything. Much like Dio's original vampire form, it far surpasses human limits, but without the weakness to sunlight.

Transmute: Diego can morph other creatures into dinosaurs as well, an ability retained from the original Scary Monsters. Diego typically uses this ability to create armies of miniature carnivorous raptors, or turn his own horse into one to grant it the ability to climb walls.

Fossilize: Introduced later in the story, Diego demonstrates the ability to not only turn other beings into dinosaurs but, if he wishes to do so, alter them into a hardened dormant state much like a fossil. The ability can be used a variety of ways, from fusing a being into a structure like a rock or tree as a camouflage (not unlike the dormant state of the Pillar Men) to shaping the fossilized being into something more creative like candle holders.

Hand to Hand: Perhaps the most curious and unique ability of Dio's power. Being bound to his body, Scary Monster allows him to fight in first person against other Stands and physically damage them, whereas normally Stands can be damaged only by other Stands.