Well, I can’t recommend members of the abra line enough. Although abra is a bit difficult to train at first (what with the fact that most wild-caught ones can only use Teleport until it either evolves or undergoes special training via TMs, HMs, or tutoring), it grows quickly, it’s loyal, and it’s highly intelligent, qualities which can make it a ready and capable partner early on. Moreover, it’s quite powerful in its own right (especially when its final evolution is allowed to mega evolve), but by itself, an abra can encourage a trainer’s growth by forcing them to think creatively on the battlefield. Let’s just say that Teleport is far more useful than one would imagine at first glance.
Alternatively, members of the gothita line fill a similar niche on the battlefield (although—and perhaps this is my own bias in saying so—not quite to as powerful a degree), as do starmie and, believe it or not, mr. mime. For those with the same temperament as members of the abra line, you may wish to consider gallade (which would also make an excellent partner to your gardevoir), meditite, jynx, or woobat.
On that note, as many psychic-types can be a bit fragile, you may find some luck in adding a member of the solosis line to your team (yes, even though you have the steel-type metagross to support you). Members of the solosis line are notoriously bulky, and as such, they’re capable of taking hits just as much as they are dealing them. Slowbro is also capable of filling a similar niche, but the problem is that members of the slowpoke line (except slowking, which is difficult to obtain) are a challenge to raise—not because they’re disobedient or unruly but instead because they’re highly capricious and, well, slow to respond to commands. That and slowbro works best when paired with a mega stone of its own. However, with persistence and a mega stone, slowbro has the potential to fill its niche better than reuniclus can, so one should think carefully about trade-offs if they’re considering a defensive psychic.
If you’d prefer speed, look no further than an Alolan raichu. Few psychic-types can outspeed an Alolan raichu surfing on its own tail, but it’s vital to remember that a pikachu must evolve within the regional boundaries of Alola to attain its half-psychic form. Even pichu and pikachu raised in Alola all their lives will evolve into their standard forms if removed from Alola and exposed to a thunder stone anywhere else in the world. This, of course, makes obtaining an Alolan raichu rather tricky, as adopting one already evolved can be quite expensive for this very reason.
Finally, should you simply want something easy to train, the fennekin line evolves into the psychic-type, and starters are among the easiest pokémon to train, as they were bred specifically for that. Unfortunately, the downside is that delphox isn’t quite the strong pokémon you may be looking for.
If all else fails, eevee can evolve into the psychic-type espeon.
Bill, you can’t keep suggesting eeveelutions whenever someone asks you for help on building their teams. —LH
Watch me. —Bill