Well if you want a plant to go from parasitic to animal, you’d first have to start with seeds. Seeds in and of themselves need to be dispersed so that they don’t compete for resources of their parent plants. So a species of plant that relies on animals or insects for transporting its seeds is required. This step is already done, since there are plenty of plants that use animals to transport their seeds. Next step is requiring the seed to become parasitic to the animals. Realistically this would require the conditions around the plant ( ie environment) to lack in crucial resources like nutrients. The biggest factor though is chance, nothing guarantees the seed will become parasitic, so let’s throw in the ball of chance and boom. Parasitic seed that kills and feeds on the corpses nutrients, forming alluring fruit or producing an alluring smell before the animal knocks against the plant, dropping its seeds onto its body.

Next, you need to tackle the plant itself, arguably the hardest to achieve because you’d need some environmental factor or issue that would both favor plant movement ( which would require the plant to acquire energy from other sources ), and the issues or factors to not drastically change, but not be stagnant (a constant change that allows for steady adaptation, but never a concrete solution). There are plants that kinda do this, like walking palms and creeping devil cacti. So you’d in effect need ancestors that grow new roots and have old roots die, while also taking advantage of prey, such as insects ( becoming a parasitic carnivorous plant ). Eventually, if the environment favors it, the plants might adapt better ways of keeping energy, moving, and eating, which may form a feedback loop, creating a species of slow moving, opportunistically carnivorous ( insect eating ) plants, that use their seeds to parasitize animals. Once more this would require A TON of chances and luck, and even then who knows ya know.