This is an idea I just got, and I'm writing it
How did Don break his tail? Belladonna took Donatello to a network of caves where a herd of Wild Humanimals had lived for generations. They stayed and ate and enjoyed their hospitality and were given a cave of their own to sleep in…In the middle of the night Don had to take a whiz so he slowly plodded to a designated latrine area (I'll refrain from describing it) As Don was exiting the latrine cavern he hadn't yet closed his 'plastron thong' the bit of shell that goes between his legs preventing any indecencies (And also prevents evil Ninjas from attacking your unmentionables) In order to properly close the Turtles' tails are kept under the plastron thong which is why for the longest time their human friends April and Casey were under the impression the Turtles didn't have tails.
But tails they did have, and as Don took a moment to stretch and yawn, two young Humanimal Children (Probably of the nocturnal variety) were chasing each other in some late night horseplay, not watching where they were going they collided into Donatello who fell sharply in his tail.
"OW!" He screamed, tears squirted from his eyes as the sharp pain in his tail sent shock waves through his entire body!
The two cubs watched in horror and the Mutant Turtle stood and his tail…Oof! His tail appeared to be best in several places! It had taken the appearance of…A… W…
The festivities were promptly ended, Ichabod was quickly called to apply an electronic cast (An electronic cast utilizes nano-macines to do things like mend broken bones and can even replace an amputated limb! The process is not instantaneous however the more severe the wound, the longer you'll be in the cast)
Don grimaced as Ichabod applied the cast to his tail, from the far side of the room he could hear his brothers snickering
"…Not…Funny…Guys!" Don grunted as Ichabod applied the devices that would keep the E-Cast strapped to his ass.
"C'mon Donny!" Mikey said "You know this'll be a funny story later…You know…Once you get the feeling back in your tail."
Don couldn't imagine being anything but mortified as the cast on his tail meant he couldn't close he shell thus he was temporarily completely exposed, and there was no way he would be leaving the confines of the Rocky Mountain Nature Research Station (That was built near the Wild Humanimals' caves to help protect them and also to study some strange phenomena that had been happening near the Rocky Mountains as of late) in anything less than an absolute emergency!
"You need to stay put." Ichabod said as he placed the last adjustment "Avoid unnecessary movement…"
"I won't be going anywhere with my shell open like this." Don said
"I'm real sorry about your tail." Ichabod said
Don sighed irritably and lay down on his stomach on the sofa. The long circular sofa of the Research Station's Rec Room could fit twelve humans or creatures as large as humans or four or five larger than average creatures like Humanimals like Pachyderms, Giraffes, Large Species of Dinosaurs.
This is what I got so far…This story is a bit experimental