Eat excrement to be able to transform into the creature whose excrement it is. Transformation is not permanent but some characteristics: mental or physical may remain if the magic is practiced frequently.
Eg: eat a goat pellet, have the ability to transform into a goat, specifically of the same breed as the goat whose pellet you swallowed.
After the transformation has ended, some temporary mental or physical effects may remain, for a time. But slowly & surely, if the practitioner keeps using that transformation again & again, characteristics take a deeper root. In the case of the goat practitioner, it could take the form of growing horns or hooves, multiple stomach chambers and need to eat grass or worse a compulsion to eat grass without the ability to completely digest it.
In case that a practitioner specifically utilizes different transformations each time, the common characteristics between the animals start appearing. Regardless, too much use can wear down a practitioner's human intelligence. These effects don't necessarily have to be negative, entranced strength or senses or agility or longevity may also be achieved. But negative effects are just as likely: skittishness, desire to eat only grass or insects or a need to hunt, lack of inhibitions or becoming more easy to anger, loss of colour vision, animal instincts, inability to read human behavioural or social cues, different muscular structure, horns, inhuman eye colours, cloven hooves, tails, scales, body hair, etc.
Staying in a transformed state too long might aggravate these problems, you might also get stuck or forget you were ever human or both.
offering of food & drinks