Yeah, I could believe this origin story.

Reupload

@Background Pony Number 17  
“Clyde Dear we need a present for Diane pies birthday,we gave all her sisters presents on their birthdays .”  
“I know,I know but what? We already gave everypony else rocks and only maud really liked hers…dang it what can we give her on a rock farm!”  
(Knock knock!)  
“?…yes?”  
“Oi there mate!Are you the no doubt proud proprietor of this here magnificent geode farm?”  
“That’s me names Clyde pie’…are you here to buy a rock or-”  
“Oh no sir I’m here for you!For you see on my flank lies the mark of hunter of various beasties and I’ve decided to sell me stock or perfectly safe critters to those willing to let a new happiness enter their lives!Take a look at this bloke(pulls out egg) this here’s an alligator egg fresh from his mamas nest (don’t worry mate she’s being worn on by fancy mare in hoofington now hehe)very ripe,liable to hatch tomorrow by estimate and since I don’t have room in me wagon I’ll be willing to give him(yes I know its a him cutie mark remember)along with a instruction guide and supplies to you and yours for one bit. Waddya say to that mate…”  
“….hatches tomorrow you say?”  
“Sure as a koala ain’t a kangaroo”  
“And it’s harmless…”  
“Well a baby alligator shouldn’t be a threat to any pony. This species won’t grow in teeth for years and are very passive and can last for decades especially here on an open farm with lots of room and little critters to eat(you farmer types are used to that stuff right?With the chicken and-eh rock culling and all)and feeding habits and the like are all touched in on me book man eh ah Sir…all for a bit.I’d take the deal…”  
“Clyde…”  
“I’ll take it!”  
Note:despite this writing I still personally believe that gummy was a gift to pinkie by Surprise(pinkies granny pie)much to the befuddlement of her parents.

In “Party of One”, Pinkie said Gummy was born a year ago, and his birthday is a day before hers. I’ve always assumed Pinkie got him new-hatched as a birthday present.

Crocodillians are born with teeth, and constantly lose and grow new ones over the course of their lives. If Gummy never grew any teeth even a year after Pinkie found him, He’d never survive in the wild. He’s a lucky little critter.
 
Also, this is adorable

Nopony understands Gummy like Pinkie Pie.