A few ideas for you, Pips. I think maybe what you intend is a hybrid of these?
Deceptive Shapeshifting, also called Deceptive Assimilation or Batesian Mimicry is the ability to shapeshift into a form that appears dangerous or menacing, while only retaining the abilities of one's original form. One with this ability could turn into a Dragon, but would be unable to fly or breathe fire, and their tail would be limp (unless any of those abilities applied in its true form). This ability is not for combat, simply for a mode of defense (like transforming into a snake, but having no venom).
Deceptive Form, also called Insidious Form or Insidious Mimicry is the ability to assume a form deceptive to the eyes, or any other sense. This ability may include the power to assume a form similar to a kitten, a puppy, a baby, a more attractive version of oneself, or the like, when in fact, the user may actually be a dangerous, maniacal killer (the user would maintain normal thinking and motor capacity), etc. This form may indicate that the user also has the power Feigning, to fool foes into thinking the user is helpless or defenseless (this ability also helps if the user’s natural form is ugly or unappealing to sight).
Monstrous Form is the ability to assume a form which is far more grotesque and frightening than one's normal form. This form has ugly or scary features, but may also have far-enhanced versions of any other abilities the user has in his normal form (this form is also prone to have the ability Psychic Intimidation). This form may be beastly (like a humanoid animal, or with strange skin coloration, or markings/features), alien, or just the user's appearance distorted (sometimes larger or smaller than one's normal appearance).