I think that, more than likely, they already knew a bit about him prior to when he and Ruby got there.
My theory is that maybe Littlefoot at least told Grandma and Grandpa Longneck, who may have asked him why he disobeyed them and didn't stay back in film II, and he told them then.
If not then, then I think it highly likely that the grownups asked tons of prying questions between when they arrived back with their families in V and when they returned to the Great Valley at the very end of V. It is even not beyond the realm of possibility that the Gang of Five had Elsie bring over Chomper on her back. (He could have stayed on her back in case the grownups might try and harm him, though I think, to be honest, he'd never have thought of that, as he totally believed his parents wouldn't eat his new friends, so he may not know anymore than that Leaf Eaters don't like to be around his kind and maybe why (I think his parents may have told him some stuff).
Also, Elsie is also helpful as to how Chomper could get off the island if need be later. However, why he would be threatened by Red Claw out there. One surmises that, and this kind of makes sense from what I read, as it seems first the whole path was gone, but more and more of it starts to come back if you look closely. Perhaps it was only temporarily washed out.
Another thing is that the Gang of Five, minus the first day, only stayed on the island the day they were with Chomper. There may be another way to shore, even if the Gang of Five didn't know about it. Yes, I know the term "island" was used in the movie title, but it doesn't mean it might be, though too deep for the Gang of Five to cross, shallow enough for Chomper's full grown parents to wade across and take him on their necks or whatnot.
Also, there is also the possibility of a D-Day low tide that could help with the getting across.
(Heck, maybe there is a mega tree the size we saw in the next movie that Chomper's parents were able to smash and float across on. It seems likely as Chomper's parents wouldn't bother to eat green food on the log and cause it to go crazy.
)
Anyway, it seems odd that they wouldn't know about Chomper till Ruby came nor that the Gang of Five, who, we can guess, never heard of Ruby till then, would trust her and not think she wasn't bringing a fake Chomper to do harm or something. (Would you trust a Fast Runner you know nothing about?)
My best theory still holds that they learned of him in around the time of film V. It makes the most sense. What are your thoughts?
