Pikachu may be a formidable opponent during official matches since Pokemon's debut episode, but the fluffy yellow Electric-Type has been known to stir the pot off the battlefield too. As Ash's first and favorite Pokemon, Pikachu has a bit of an ego about him, and his pride has caused some unnecessary conflict in the past.
However, there have been times when Pikachu's hostility is entirely warranted, in moments when his friends are put in danger, and he goes into protective mode. All in all, most of Pikachu's rivalries provide some entertaining comic relief and allow viewers a glimpse into the unique interactions between Pokemon, emphasizing their individual personalities.
Cat and Mouse
With a rivalry as old as the Pokemon franchise itself, Pikachu vs Meowth is a contemporary spin on the classic Tom and Jerry tale, as the feline Pokemon consistently pursues the yellow mouse, and is comically thwarted at every attempt. What irks Pikachu more than the constant conflict is the fact that Ash is regularly endangered during their outlandish schemes, and poor Pikachu is often restrained and unable to assist.
In "Pokemon Shipwreck", Pikachu's hatred for Meowth is emphasized when he snaps his fingers in disappointment when Ash discovers that Team Rocket had not died, and makes no attempt to assist the half-drowned cat. However, after time, Pikachu slowly starts to warm up to Team Rocket and is often seen conversing with Meowth. His newfound affections are affirmed in "Pikachu Translation Check," when Jessie and James gush over all the moments Pikachu showed kindness to his rivals.
Evolutionary Rivalry
When it comes to Raichu, Pikachu does not discriminate and seems to hold great disdain for all of his evolved forms along the way. The first memorable Pikachu/Raichu standoff occurs during the Pokemon Indigo League, where Ash faces off against Lt. Surge and his egotistical Electric-Type Pokemon. After Raichu showed no mercy in their first battle, Pikachu comes back with a vengeance and overpowers the arrogant Pokemon, earning Raichu's respect in the process.
Pikachu had an equally difficult time against Sho's Raichu, whose attacks landed Pikachu in the Pokemon Centre for treatment (but no medication could expel his embarrassment). This public loss initiates an internal conflict within Pikachu, who starts to question his decision not to evolve. Thankfully, Team Rocket intervenes and steals the Thunderstone from Pikachu before he could use it, and later cheers him on as he defeats Riachu yet again in "Pika and Goliath!"
Fans were reminded of this epic rivalry once again when Ash and Pikachu head off to Melemele Island and come face to face with Noa and her hovering Riachu in "Racing to a Big Event!" Things get really serious as the Electric-Type Pokemon takes their competition to the next level in the Great Alola Pancake Race. Although Ash and Noa train together, they do not inspire a friendly competition within their respective Pokemon, whose animosity is palpable. However, in a surprise turn of events, the Electric mice have to share second place as the sleepy Komala sneaks in to win a year's supply of pancakes.
Insults and Imitations
Pikachu did not instigate this rivalry, however, finds himself at the receiving end of much hostility from Jessie's Mimikyu. The Ghost/Fairy-type Pokemon's hatred seems to stem from jealousy, as Mimikyu wishes he was just as loved and celebrated as Pikachu is. Lacking that distinctively cute quality that Pikachu possesses, Mimikyu hides its face permanently under a costume that attempts to mimic Pikachu's aesthetic.
Its disdain is so fierce that Mimikyu struggles to focus during battles, often aiming its attacks at Pikachu, regardless of the intended opponent. In an attempt to try and get her Pokemon to let go of its hatred, Jessie tries to encourage Mimikyu to adopt a different disguise, but it shows no interest in making a transition.
AC/DC Action
Another example of Pikachu's strife with an entire group of Pokemon is his attitude towards Electivire and any of its evolutionary forms. Electivire is considered by many to be Pikachu's biggest Electric-Type opponent (of another species) and these two constantly butt heads while attempting to prove their superiority.
Pikachu has a distinct disdain for Paul's Elekid when they first meet in "When Pokémon Worlds Collide!" but by the time Paul's Pokemon evolves into Electabuzz, the Electric-Types seem to have settled their differences. However, this didn't stop them from battling as Electivire proves to be the more powerful Pokemon in "Battling a Thaw in Relations!" Pikachu manages to redeem himself when he battles against Voklner's Electivire in Pokemon Diamond and Pearl, winning the match for Ash and elevating his status in the World Coronation Series.
Sunglass Skirmishes
Who could forget the biggest rivalry between Ash's Pokemon, as Krokorok completely rubs Pikachu the wrong way, making teamwork all but possible. The wild Sandile is a constant trouble maker who regularly instigates fights with Pikachu, and not much changes once he joined Ash's team.
He appears to admire Ashe greatly, as the Dark/Ground-Type Pokemon follows the trainer along his adventures, observing from a distance. It seems that by the time he has evolved into a Krookodile, the Pokemon no longer has anything to prove to Pikachu, and the animosity between the two eventually fizzles out.
The Sinister Scorebunny
These two first met during an unofficial battle, after a disguised Scorebunny had stolen Ash's backpack in "Settling the Scorbunny!" and gets aggressive with the young trainer. Pikachu uses his Electroweb to restrain the angry Fire-Type rabbit, as Ash tries to gain control over the situation. Once Goh correctly identifies the Pokemon, the boys try to have a polite conversation with him, but Scorebunny is relentless and continues to attack Ash. Pikachu steps in again with a Thunderbolt to shock him into submission, and Ash gets electrocuted in the process.
Scorebuny takes a while to warm up to Ash (or anyone for that matter) and due to his repeated assault on the trainer, things are tense between Scorebunny and Pikachu. The distance between them intensifies when the Fire-Type evolves into a Raboot, but by the time he is a Cinderace, the pair of Pokemon are as thick as thieves!
Pikachu the Pugnacious
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For such a cutie-pie, Pikachu sure has a mean streak, often reminding fans of the unfriendly nature this Pokemon has displayed since the very first episode. Pikachu seems to assess each new addition to the group based on the vibes he receives from them, and can often either be immediately hostile or amicable and welcoming. When looking at the bigger picture, it is important to note that most of Pikachu's rivalries spark from his protective nature, and have a lot to do with defending Ash's reputation as well as his own.