Except, as it turns out, she never cared about that — she was, however, jealous of Rarity's relationship with Razorwing, and tried to get them to break up so she could have Rarity for herself. This backfired horribly, and ended up leading to her friendship with Rarity falling apart.
In the My Little Pony: Friendship Is Magic fanfic A Delicate Balance, Fluttershy isn't at all happy that Twilight's infatuation with Applejack means that they can't spend as much time together. Fluttershy being Fluttershy, she feels awful about the fact that she's putting her own feelings before Twilight's and tries to be supportive of Twilight's relationship anyway.
Sakura in A Growing Affection is none too happy that Naruto moved on and started dating Hinata. She does admit to Lee, however, that it might be that Laser-Guided Karma finally caught up to her.
(bitterly) "It's funny. He (Naruto) used to like me but I was so busy trying to get Sasuke and he made me miserable. Now I like him but Naruto is with Hinata and they're happy."
Grimlock in the Transformers fanfic A Child Shall Lead Them. He is not happy when fellow Dinobot, Swoop, ends up getting the Matrix and becoming leader of the Autobots, the position he's always coveted. He gets better and ends up serving as a mentor figure to his teammate.
Kakashi calls Sasuke out on this in Legacy Of The Rasengan:Naruto. Despite knowing about Naruto's life Sasuke is jealous of Naruto's power. Kakashi points out that A) Sasuke is being very childish (because Naruto has something he doesn't), B) Sasuke knows for a fact Naruto only uses the Kyuubi's charka as a last resort and C) because of B, Naruto works and trains harder than anyone in his age group going so far as to be a Combat Pragmatist and Obfuscating Stupidity to get the drop on people.
Hefty is this to Empath in Empath: The Luckiest Smurf, especially when it comes to Smurfette, who passed over Hefty when he originally proposed marriage to her along with his fellow Smurfs because she couldn't decide which Smurf she loved the most at the time.
Trixie herself has jealousy as one of her main character flaws in this verse. In the original fic, she shows jealousy over Twilight's close relationship with Shining Armor compared to her relationship with her elder sister and in Perfect Companions she becomes jealous of the rest of the mane six having pets when she doesn't. She also shows jealousy of Twilight getting to perform in front of Celestia and the Saddle Arabian delegates. However, she has a lot better control over it than she used to before her Heel-Face Turnor her insane sister has and doesn't allow it to override her loyalty to her new friends.
The source of Daryan's abuse of Klavier in Dirty Sympathy: Daryan was originally their band's singer and frontman before their label made them switch. The switch became permanent when it was proved that Klavier had better stage presence and better vocal range.
Robin becomes jealous of Sumia in Pretender for having Chrom's love and for being married to him. Although he feels very bad that he does and overcompensates by making sure Sumia is well protected in the battle field.
Sumia is jealous of Robin for taking all of Chrom's attention despite them being recently married and Sumia about to give birth to their first child. She knows that Robin has feelings for Chrom and Chrom keeps trying to get Robin's attention.
Chrom become jealous of Frederick for seemingly taking his place as Robin's closest friend when Robin starts to become distant when he moves out of Ylissean palace after Sumia and Chrom's marriage.
In Graduate Meeting Of Mutual Killing, Ginchiyo was jealous of her sister for receiving the credit and fame for their work. This led her to try to usurp the invitation to Hope's Peak, but when it's revealed it was meant for both of them, she goes back home to tell her sister. The consequences were much more drastic than what she expected.