This is a list of all the short films related to the anime TV series Pokémon, whether I've seen them or not. I guess they're actually called "Pikachu shorts," but I prefer to call my page "Pokemon shorts" just in case there are ever any non-Pikachu shorts in the Pokemon franchise... which I'm fairly sure there currently aren't (unless you count The Birth of Mewtwo), but I could be wrong.
Note: for most shorts (excluding the first one), I'll use the title of the American release. However, when a short has no American release, I'll use Bulbapedia's English translation of the Japanese title.

1. Pikachu's Summer Vacation (21 minutes)
ANN; Bulbapedia; IMDb; Serebii.net; TV Tropes; Wikia

This was released in 1998 in Japan (and 1999 in the U.S., as "Pikachu's Vacation"), alongside the feature film Pokemon: The First Movie. (I think I saw it on Cartoon Network, when they aired the feature movie. I know there are some VHS and DVD copies of the movie that include this short and some that don't, and I'm not sure if the short is included on any sites that stream the feature.)

Ash, Misty, and Brock's pokemon all take a vacation together without the humans. Pikachu's supposed to look after Togepi, who of course gets into all sorts of standard-issue cartoon trouble, though Pikachu's the one who suffers. Then the group encounters some other pokemon and square off against each other. Then Charizard's head gets stuck in I don't know what it was supposed to be, but they all work together to free him. The song at the beginning of the whole thing was catchy, and the scenes are all separated by some trippy segues. It wasn't great, but it didn't suck, either.

2. Christmas Night
Bulbapedia; IMDb; Pokemon.com; Serebii.net; TV.com; Wikia

I guess this was originally released on home video, in Japan, in 1998. Its release history on home video in the U.S. is too complicated for me to follow, but it was eventually included as half of the Pokemon Chronicles episode "Pikachu's Winter Vacation" (along with the next short, "Kanga Games"), in 2006. I saw that episode online at some point, but now I don't remember it at all.

3. Kanga Games
Bulbapedia; Serebii.net; Wikia

See "Christmas Night."

4. Pikachu's Rescue Adventure (20 minutes)
ANN; Bulbapedia; IMDb; Serebii.net; Wikia

This was released in 1999 in Japan (and 2000 in the U.S.), alongside Pokemon the Movie 2000. I haven't seen it, because when I watched the main movie on Cartoon Network, they didn't show the short. And I don't know if it's available on any home video or streaming format.

5. Winter Games
I haven't seen it.

6. Stantler's Little Helpers
I haven't seen it.

7. Pikachu & Pichu
ANN; Bulbapedia; IMDb; Serebii.net; TV Tropes; Wikia

This was released in 2000 in Japan (and 2001 in the U.S.), alongside Pokemon the Movie 3. Once again, all the pokemon get dropped off to have some fun by themselves. Pikachu meets a couple of Pichu brothers, and they get chased by a Houndour, and then meet a bunch of other pokemon and have some fun. Then Pikachu, Pichu, Houndour, and all the others they met have to work together to prevent some junk from collapsing, and then Pikachu goes to rejoin his friends before Ash and the other humans get back. Then there's a feast to celebrate the anniversary of the day Pikachu and Ash first met.

8. Delibird's Dilemma
I haven't seen it.

9. Snorlax Snowman
I haven't seen it.

10. Pikachu's PikaBoo
I haven't seen it.

11. Camp Pikachu
I haven't seen it.

12. Gotta Dance!
I haven't seen it.

13. Pikachu's Summer Festival (unreleased in U.S.)
I haven't seen it.

14. Pokemon 3D Adventure: Find Mew! (unreleased in U.S.)
I haven't seen it.

15. Pikachu's Ghost Carnival (unreleased in U.S.)
I haven't seen it.

16. Pikachu's Ocean Adventure
I haven't seen it.

17. Pikachu's Island Adventure
I haven't seen it.

18. Pikachu's Exploration Club (unreleased in U.S.)
I haven't seen it.

19. Pikachu's Ice Adventure (unreleased in U.S.)
I haven't seen it.

20. Pikachu's Sparkle Search! (unreleased in U.S.)
I haven't seen it.

21. Pikachu's Strange Wonder Adventure (unreleased in U.S.)
I haven't seen it.

22. Pikachu's Summer Bridge Story (unreleased in U.S.)
I haven't seen it.

23. Sing Meloetta: Search for the Rinka Berries (unreleased in U.S.)
I haven't seen it.

24. Meloetta's Moonlight Serenade
I haven't seen it.

25. Eevee & Friends
I haven't seen it.

26. Pikachu, What's This Key?
I haven't seen it.

27. Pikachu and the Pokemon Music Squad
I haven't seen it.