Beast Doom

The material component is an unbaked clay model of the animal you wish to transform the target into. This model can be crude, but a small amount of hair or any body fluid from the animal must be blended into the clay. Alternatively you can use the figurine resulting from the reversal of a previous transformation. The model needn't be held, but it must be somewhere on the caster's body, 

Roll 4d8 plus 4 times your spell proficiency. If the result is greater than or equal to the target's current hit points the target is transformed into a copy of the beast that provided the tissue sample.  A successful WIS save halves this roll. If cast at higher levels each level above 4th adds another d8 plus proficiency bonus to the pseudo-damage.

Should the roll be less than their current hit points the transformation fails, but the damage rolled becomes real necrotic damage.

These limitations can be waived by the target, if they chose to accept the transformation (including under duress). In that case no damage occurs.

If the target is on zero hit points the transformation stabilises them. 

The target is a single creature. The spell takes the rest of the round to complete the transformation. The target's body liquifies, all carried and worn gear falls through the liquid form to the ground, then the liquid forms itself into the beast, first growing or shrinking as necessary. Spell fails if there isn't space, though it will push fluids and flesh and blood creatures aside. It also fails if the beast is not viable in the environment, for example fish out of water. While in flux it's immune to all damage, and incapacitated. If the spell succeeds the clay model changes into a figurine of the target's original form, and becomes indestructible. Any body modifications, such as tattoos, brandings or piercings become part of the figurine.

Should the target be under a temporary transformation, like Polymorph or Wildshape, then the beast form replaces their "natural" form, but they can still wildshape from there, so that they become the beast when their transformation expires, they can still Wildshape if they have that ability, but not back to a humanoid form.

The resulting beast is a non-identical twin to the animal that supplied the tissue sample except that its soul, intellect, episodic memories, and it's loves and hates towards other creatures are those of the transformed person. Current and maximum hit points are as before the transformation. It's emotional reactions to, sensory inputs are those of the animal. The INT stat is unaffected, all other stats are those of the animal including AC,  sex, hormonal state and attacks. It understands languages known to the person, but it's vocalisations are that of the animal. The transformation leaves no residual magic except in the figurine.

The transformation cannot be dispelled, the effect is complete. But if the figurine touches the body of the animal the transformation is reversed, and the figurine changes to that of the animal form.  The figurine in now a high quality image of the animal, as it was just before being changed back, and it can be used as the material component of a fresh transformation. Any modifications, such as horse shoes, brand marks or even training, but not harness or the like,  become part of the figurine and will be expressed in the animal resulting from a fresh use of it.

Without the figurine only a Wish has the power to reach back in time and retrieve the animal's original form, or locate the figurine wherever it is and activate it. Lessor spells cannot detect the fact that the animal is not what it appears, except those that make mental contact with the human intellect it carries. Should the subject have reason to expect such mental contact, they may chose to hide their intellect behind the animal mind. This includes Speak with Animals.

The caster can target themselves, and revert if they can find the figurine in the dropped equipment.