In general, whenever one of your pokémon is terrified of another, it’s best to start slowly. It’s good that your pikachu and dratini get along well, as it may be the key to your dratini’s predicament. Have your pikachu mediate interactions between your dratini and sylveon. Whenever dratini shows any level of discomfort, have pikachu be there to ground it and reassure it that sylveon is a friend who would never hurt it.
Moreover, have them start with short interactions and work up to longer conversations. Think of it like exposure therapy to humans: you want to begin with only a little bit of exposure to avoid trauma, and as your dratini adjusts to that level of interaction, increase the amount of time it spends with your sylveon, bit by bit, until it can tolerate long-term exposure. Eventually, when your dratini can tolerate conversations with sylveon, begin having them fight alongside each other in double-battles to reinforce your dratini’s progress. If you’re not a trainer, have them perform tasks together around the house.
Also, be sure your sylveon is in agreement with whatever you do. That is to say, ensure that your sylveon hasn’t done anything to warrant dratini’s fear, and be absolutely sure sylveon wants to interact with your dratini. If your sylveon is just as uncomfortable with the idea of being on the same team as a dratini, this is something you’ll also need to resolve using similar gradual introduction methods as mentioned above. Your pikachu should be able to help as well. If, however, your sylveon is very eager to make a new friend, allow it to be involved in the efforts to calm dratini down. Ensure your sylveon takes things slow and avoids showing affection through touch at first. At the same time, be sure that your sylveon extends friendly, welcoming gestures—just from afar. As dratini begins to warm up to your sylveon, then your sylveon may show physical forms of affection, but it should still be careful and wary for any signs of discomfort from your dratini.
In other words, some relationships should start slowly and work up to a certain level of closeness, and that goes for pokémon too. Be cautious and careful, show your dratini that your sylveon won’t hurt it, and allow dratini to grow comfortable around your sylveon in its own time.
Best of luck, anonymous.