It is Springtime in Bikini Bottom, and Patrick is emerging from his rock and storing away his Winter "coat" (sweater), burying it like a dog bone. SpongeBob, meanwhile, proclaims that he has "Spring Fever", and greets the flowers in his yard. When Squidward steps out of his house, SpongeBob asks him if he has said hello to the flowers as well. Squidward does so sarcastically, and the flowers hiss at him, causing him to run inside, terrified.
SpongeBob concludes that Squidward has "Hay Fever" and, as an act of kindness, decides to plant him some hypoallergenic flowers. He does so, and the moment he takes his eyes off the flower to grasp the watering can, he discovers it's been eaten. Surprised, he plants another one, which meets the same fate as he again reaches for the watering can. Determined, he plants another one and doesn't take his eyes off of it for 3 days. Patrick says Hi to SpongeBob, and SpongeBob looks at him and says the same thing. But after he looks at the flower again, it gets eaten. SpongeBob then discovers that the culprit is a green seahorse. SpongeBob is fascinated and imagines him and the horse together riding, and decides to tame her.
Later, as he watches the seahorse, it is lured toward him his floral bookmark, which it eats. They become friends and begin to ride around and do things together. As Squidward is biking to work the next morning, SpongeBob races past him on his seahorse, which he has named "Mystery". Squidward then rides off a cliff and causes a nuclear explosion when he hits the ground. When SpongeBob reaches work, he leaves Mystery outside at the bike post. Two fish approach Mystery and mistake her for a kiddy ride. One suggests the other to try it out, and when he inserts a coin into her "slot", Mystery kicks him into the distance, and he blows up on impact. The other fish runs away screaming, causing Mr. Krabs to come out and see what the trouble is. He sees Mystery and mistakes her for a monster; SpongeBob tells him she is his pet, but even though she is harmless to SpongeBob and not the others, he is ordered to get rid of it and if he disreguards his order, he will be fired for not cooperating with him.
SpongeBob attempts to hide her in a cupboard in the kitchen until Patrick finishes building a stable. SpongeBob offers Mystery a Krabby Patty, which the horse begins craving after eating. After she craves, Mr. Krabs overhears this noice, and angrily comes in and confronts SpongeBob, threatening to fire him if he sees Mystery still here, but, however, he dosen't see her and leaves. After narrowly avoiding being caught by Mr. Krabs, SpongeBob realizes he has to be more careful. Squidward catches Mystery after an order goes missing, and Mystery goes on an eating frenzy while a crowd of customers became angry. Mr. Krabs finally catches SpongeBob nursing a sick Mystery in the kitchen, who is suffering from a stomach ache, now knowing that he had broken the rules. Mr. Krabs then becomes angry with SpongeBob and tells him that he plans to get rid of both, Mystery and SpongeBob. SpongeBob is responsible for breaking and disreaguarding the rule, but he pleads for mercy, explaining that Mystery is his friend and that he loves her.
Mr. Krabs, feeling sympathy for SpongeBob, tells him a story of his favorite dollar and how he eventually spent it for a soda on a hot day. He tells SpongeBob that Mystery needs to be freed because the indoors is not a place for a wild animal, and SpongeBob consents and takes the horse out the front door where he painfully asks her leave. He cries as she leaves without hesitation. Mr. Krabs comforts SpongeBob until Squidward shows him an empty safe. Realizing that Mystery ate the Krusty Krab's finances, SpongeBob and Mr. Krabs chase after her. The show ends with Patrick trying to get into a hat store, but the wooden plank nailed to his head blocks his way.
Running Gags: A bowl of onions is making Squidward cry but then gets mad when he sees the bowl. The first time the bowl is on the ledge between the register and the kitchen. The second time the bowl of onions is on the register. The final time Squidward is shown pushing the bowl under a customer so he won't be the butt of the joke anymore.
Mr. Krab's narration about his dollar is still similar to SpongeBob when they lose their beloved ones. Yet, it is unrelated to what he forces SpongeBob to get rid of Mystery. This contradicts his advice.
When SpongeBob and Mystery were dancing on TurboNick, a song from Nickelodeon's old show "The Naked Brothers Band" can be heard for an unknown reason.
Mystery's nickname, "Sir Eats-A-Lot" is a reference to Sir Lancelot, one of the most famous knights from the tales of King Arthur of Camelot.
The title of the episode is most likely a parody of 'My Little Pony".
During the montage, Mystery paints SpongeBob like in the 1997 movie, Titanic.
When a fish complains "Where's our food?" with another saying, "I'm so hungry!", it references the live-action film Animal Farm, where farm animals complain of hunger like the fish customers.
The ending of the episode, where SpongeBob chases away Mystery, references the end of the movie Harry and the Hendersons.
Among the things eaten by Mystery is SpongeBob's hat, but in a later scene, where he attempting to give Mystery a bicarbonate soda, his hat is whole again.