You'll get shut down, not only for using pokemon sprites but their actual sprites, if you alter them in someway you may be able to get away with it.
Nintendo are very strict on their copyright especially if you're going to make money off these pets.
This exactly. There is no way they would let you get away with using their sprites. I honestly don't think you could even legally get away with using something that looks similar to any of the pokemon either. They would sue you for theft of intellectual property and damages and who knows what else.
RO private servers can get away with ears until nintendo sends them a cease and desist. They wont likely be so harsh with that because its just ears and thats debatable as to whether or not they infringe because they could be similar. But if it looks like an entire pokemon, or especially is their actual sprite, you better believe they'll step in and jump on that lawsuit as soon as they hear about it.
While I would love to have them following me around, it's seriously not worth the risk trying to bring pokemon in here. I don't want to see another trickster server shut down.