There are a couple of things to consider, anonymous.
First, I would hate to say it, but service pokémon are typically trained for their tasks from birth. That isn’t to say your own pokémon can’t help you in their own way, but official service pokémon often need at least a year of task training and public training. You see, being a service pokémon isn’t just about knowing how to perform the tasks a human partner may need. It’s also about knowing how to be professional and calm at all times while on the job, and it’s about knowing how to focus, even in the most distracting of situations. For example, if a pet growlithe barks threateningly at a service growlithe, the service growlithe must know how to remain composed enough to focus on their work, not on attacking the other growlithe. Likewise, service pokémon must be calm and collected, even in settings jam-packed with people or in situations that are life-threatening to either their human partner or themselves (or both).
In some cases, a battle-ready pokémon may be taught to become a service pokémon, but to do so effectively, they must attend training with a specialized trainer, one who is not only capable of teaching a pokémon how to assist with their human partner’s disability but also how to heel and obey properly in any situation. This trainer must be certified for service training, and at the completion of the pokémon’s education, that pokémon must undergo thorough examination to ensure they can work reliably at all times. Only then may a pokémon be awarded the license to be a service pokémon. However, it’s also worth it to note that not every region allows for this process to happen with battle-ready pokémon, as battle-ready pokémon often react to situations with a battlefield mindset (that is to say, by attacking or preparing itself to attack), even years after retirement. Among the regions where you can train any retired battle-ready pokémon to become a service animal, you have Unova, Alola, and Kalos. For all others, it’s highly recommended that you double-check your local regulations.
That having been said, though, most species can be service pokémon. The only exceptions are cases in which the pokémon would pose a threat to surrounding humans (such as grimer, muk, and cryogonal) or are far too large to be usable (onix, steelix, wailord, gyarados, milotic, and Alolan exeggutor). Pokémon that have difficulties traveling on land (most aquatic pokémon) as well as pokémon that are made up of materials that may be detrimental to their environment (slugma and magcargo) tend to have varying regulations associated with them, depending on the region. In Hoenn and Alola, all of these pokémon are perfectly legal, but in Sinnoh, they’re barred. Of course, keep in mind that if you’re traveling to other regions, the only thing that matters if where your pokémon got its papers from, not whether or not your service pokémon is legal in the region you’re currently in. For example, if your service pokémon is a goldeen, you can travel anywhere you’d like with it. It’s just that if you live in Sinnoh, you can’t get a goldeen service pokémon. Also, you may have difficulties, as Sinnoh is a primarily mountainous region with an abundance of land routes that your goldeen may struggle with, but the point is, your service goldeen will still be legal.
So in conclusion, it’s possible to retire your pikachu and train it to be an official service pokémon. However, it really depends on which region you live in and whether or not you’re willing to have your pikachu undergo a full year of training. You can still travel with him, however, as it’s possible to set him as pet or companion pokémon, or one that you’re incapable of using for battle, so if you’re not willing to go through all that hassle, that’s an option as well.
The Pokédex According to Bill 