Was the choice of 'Mimbulus Mimbletonia' as a password a coincidence?

Question

In book 5 the Gryffindor password is mimbulus mimbletonia, the plant which Neville has just been given for his birthday.

"Harry, I know it!" Someone panted up behind him and he turned to see Neville jogging towards him. "Guess what it is? I'm actually going to be able to remember it for once-" He waved the stunted little cactus he had shown them on the train. "Mimbulus mimbletonia!"
_{(Order of the Phoenix, Chapter 11, The Sorting Hat's New Song)}

The passwords are picked by the Fat Lady, as Sir Cadogan chose his own assortment of passwords when he was standing in for her. Yet mimbulus mimbletonia is a very rare plant, apparently only to be found in the Middle East.

"It's really, really rare," said Neville, beaming. "I don't know if there's one in the greenhouse at Hogwarts, even. I can't wait to show it to Professor Sprout. My Great Uncle Algie got it for me in Assyria."
_{(Order of the Phoenix, Chapter 10, Luna Lovegood)}

It seems strange that the Fat Lady would pick a plant that Neville happened to have acquired over the summer as the new password.

Is there an in-universe reason for this or is it just a coincidence?

Answer 1

There appears to be No in-universe reason that the password was Mimbulus Mimblentonia

Harry had reached the end of the corridor to the Gryffindor common room and come to a halt in front of the portrait of the Fat Lady before he realised that he did not know the new password.

The password must have been set before or during the feast, because Harry moves immediately from the feast to the common room, appearing to beat Neville there, yet Neville already knows the password, so the Fat Lady must tell someone, professor Mcgonagall probably, who passes the password onto the prefects and others.

So unless Mcgonagall sets the password at the feast, at student suggestions which goes agaisnt our current understanding Who decides the Common Room passwords at Hogwarts?, she most likely comes to the feast knowing already what the password is. Showing that it was not due to Neville in universe that the password is Mimbulus Mimblentonia.

So we must assume its simply some foreshadowing/planned coincidence by JKR that allows Neville to tie in his plant that hes so proud of and be a help to Harry at the same time. This may be do to Nevilles increased importance starting in this book, with him being a prominent member of Dumbledores Army, and Staunch Harry supporter.

Similar Did JKR make some kind of an inside joke about Moaning Myrtle's death?

Answer 2

Probably.

There is no indication that Aligie or Neville helped select the password.

As for the issue of the plant's rarity, it would seem that, despite its rarity, the Mimbulus mimbletonia may be decently well-known in the wizarding world.

“And yet,” said Hermione, coming out of her reverie, “I doubt you’d find a woman who sulked for half an hour because Madam Rosmerta didn’t laugh at their joke about the hag, the Healer, and the Mimbulus mimbletonia.”

—Harry Potter and the Half-Blood Prince

Of course, Ron has seen Neville's specimen multiple times, but the fact that he expected Rosmerta to get the joke suggests that the plant is common knowledge.

The plant may be sufficiently rare that there isn't one in the greenhouse at Hogwarts, but given its propensity for spraying people with goo at the slightest provocation, it may well have a reputation that belies its rarity.

As such, while it certainly is an unusual coincidence that the Fat Lady happened to choose this plant as the password, it is perhaps not so extraordinary as its rarity would suggest.

_{However, it is possible that Dumbledore suggested the password to the Fat Lady, if we consider the possibility Great Uncle Algie had just brought back another Mimbulus for Hogwarts}