Odd creatures from another world that are composed entirely of living data, somewhat like Porygon. Their data format is oddly volatile, and they are able to evolve and devolve at will for the most part, as well as "fuse" with other beings, Digimon and not, and create new "evolutions". Because of this particular variable data, which is much more complex than Eevee's unstable DNA, they can only be stored in PC Storage Systems with the Digital expansion pack installed.
While Digimon data is Poké Ball-compatible, the capture rate is extremely low, and a special Ball was created with a larger storage capacity and modified encoding format to accommodate the expansive data required for a Digimon's fluid evolution ability. The current version of the Pokédex is able to register Digimon and track capture data properly, but can only display a Digimon's data type, Vaccine, Neutral or Virus.
A Digimon's baby evolutionary stage communicates just like a Pokémon, but when they evolve permanently to their basic stage, they gain human speech and a sentience closer to Pokémon such as Ninetales, Lapras and Yamask, who can understand human speech and most human concepts. The capture of a Digimon remains, however, essentially the same. Digimon will only show themselves to humans they like after a short observation period, and intend to be captured by their "chosen" human.
Strange creatures from another world that generate a constant low level of electricity and emit weak radio waves constantly. They can be communicated with by using cell phones and dialing a specific monster's "phone" number, and, due to their electrical wave nature, can be stored in PC Storage Systems with the E-Wave expansion pack installed.
E-Monster data is compatible with Poké Balls, but have such a low capture rate that a special Ball was created with a modified encoding format to provide a better capture rate. While the current version of the Pokédex can register them as EXX numbers and properly track their capture data, it tends to glitch slightly and constantly reads an secondary type from their electrical field, and records their thoughts as a Pokédex entry.
E-Monsters are different from Pokémon in that they are perfectly capable of human speech and think just like humans. The International Pokémon League advises trainers who come upon E-Monsters not to confuse them with Pokémon, and to wait for an E-Monster's offer to join them instead of capturing them. While Pokémon view humans as people to grow stronger with and put their faith into us on capture, E-Monsters judge friendship just as we do, and will not obey a trainer whose personality is grating or displeasing to them as an individual.
Mistreatment of E-Monsters is also noticeably different from Pokémon abuse, in that E-Monsters are entirely aware of their strength relative to a human's. Most E-Monster abuse incidents have left the trainer with a painful bruise or two, and some crushed equipment that was used in the E-Monster's capture, and left the particular E-Monster understandably grumpier for a time.