It is possible that there would be ponies who are naturally aware of changeling tricks and practices, and who would resist consumption on the emotional level, but perhaps not the physical.

Papi will only convert a pony if they are willing to join under their own volition upon discovering the hive, and if that is not an option and they prove no great threat, then their memories are altered and they are escorted out of the hive.

Even if a pony can resist a changeling’s natural wiles, there are few who can resist upper level hypnotic magic.