This curse does not affect constructs, elementals, or other inorganic life.
Shapeshifters can end this spell and change back to their normal form as an action, or possibly faster.

This spell does not take effect until the start of the target’s turn.
At the start of its turn, the target gets a social save.
(A creature that wants to can choose to fail its save.)

Fail:
The target transforms into a tiny, small, or medium animal of the caster’s choosing.

While they are transformed into an animal, they cannot use weapons or cast spells other than Beast Within.
They may make an unarmed melee attack, which deals 1 weapon damage.
If they are slain while in this form, the creature returns to their normal form, with whatever HP they had at the time of their transformation.

Powerful Creatures:
Powerful creatures, such as bosses and adventurers, get a social save to transform back at the start of each of their turns.

You can end this curse as a fast action.

Pass:
The target has disadvantage on rolls to hit and tests of skill until the end of its turn, as it struggles to control its body while this spell surges through it and tries to alter its form.
Shapeshifters (including adventurers with Beast Within) are unaffected if they pass their save.

Suggested Backfire: You begin to gain newt-like features. At the start of your next turn you must pass a save against this spell or turn yourself into a newt (and you cannot dismiss that transformation).
Or the target turns into a formidable monster and you cannot undo the transformation.

A target can only be affected by one Turn Into a Newt spell at a time. If the target fails its save against a new one, the old spell is overridden and dispelled, and the target is transformed into the new form.