I read this book when I was a pre-teen, definitely before 1988 and probably before 1984, in English, in the USA.
I think that the book may have been some sort of "Choose Your Own Adventure" or "Pick a Path" book, maybe even a "Time Machine" story. It's definitely not from a Gamma World RPG novel or TSR product, although it may have shared some stylistic elements.
The only thing I really remember from the story is that the main character (a young or teenaged boy) was walking through an ancient overgrown forest/jungle, and he encountered a Tyrannosaurus Rex that had a long trunk, like an elephant.
I think there may have been an illustration, too, showing the creature, but I'm not certain of that.
I don't think that the creature featured prominently in the tale; as best as I can recall it was a brief throwaway encounter, the creature showed up, scared the main character, then was never seen again -- although I suppose I should say that I'm not sure about that either.
I'm sorry that there's so little to go on, but it's my hope that a "T-Rex with an elephant-like trunk" is a memorable enough detail to ring a bell for somebody.