We thought something, but something else has happened (恋する獅子丸でござる！の巻) is episode 13a of the Ninja Hattori-kun 2012 anime.
Shishimaru is smitten when he meets a poodle.
Hattori is training Shinzo and Shishimaru by throwing dumplings at them, when Shishimaru catches sight of a poodle. He is smitten and loses concentration, resulting in him getting hit in the face by a dumpling. As Hattori lectures Shishimaru, he sees the owner of the poodle, calling her to the name of Marine.
Back at home, Shishimaru is still thinking about Marine, and refuses to eat much at dinner. The next day, when Hattori tells Shinzo and Shishimaru that they will be training near the mountains, Shishimaru tells him that he wants to train near the stream. Hattori grudgingly complies.
At the stream, Shishimaru is training when he sees Marine again. When Hattori and Shinzo ask him what's gotten into him, they see Marine and her owner. When Shishimaru tells them that he's smitten with her, they turn to Kenichi for advice.
Having met Kenichi, they tell him about his situation with Marine. Kenichi says that first of all, he should look good, before doing a makeover on him. He is humiliated when the gang laugh at his makeup. Eventually, when Shishimaru worries about his appearance, Hattori uses his ninja techniques to change Shishimaru's appearance, and Shishimaru shows off his new muscly self.
Later on, they go back to the stream, and meet Marine and her owner. While Kenichi talks to her owner, whose name is Rumi, Shishimaru shows off the effects of his bodybuilding to Marine, but falls over.
Back at home, Kenichi tells Shishimaru that merely looking good doesn't help, and he has to act with dignity in front of a girl. He also suggests he give her a gift, and Shishimaru chooses to give her... chocolate rolls of course.
A little while later, Shishimaru sets off to the stream with a chocolate roll, but on his way there, Kagechiyo the cat nicks it and pounces off.
Back at home, Shishimaru goes to his kennel and wallows in self-pity. Hattori encourages him to come out of his shell, which he does. As he trots off to meet Marine, he sees her, along with Rumi, being threatened by a pack of strays. He knocks one of them out with a bone, but has trouble tackling the others. Thankfully, he pulls through, and the dogs escape. As Hattori and Shinzo watch on from a roof, Shishimaru talks to Marine, but gets a shock when he realises that Marine is a boy who has been over-mothered by Rumi!