The Monstrous Arcanum book is a supplement released by Forge World, it's official part of 8th edition Games Workshop lore but like everything Forge World sells, it's expensive.
Basically you have to talk it with your opponent first. You cannot just show up with a Dread Saurian in your list to an official event with permission. But most players will allow you to use Monstrous Arcanum stuff if you ask nicely, especially if you have a cool model representing whatever you plan to use.
Technically, if you play a
Storm of Magic game, you can use Monstrous Arcanum monsters without obtaining special permission. But I haven't played a lot of Storm of Magic games. I only played at one tournament at a game store but it kind of fizzle. We each had three matches and each match was supposed to take an hour. Guess what? An hour is not sufficient for a 2000 point Storm of Magic match. I don't think I ever managed to finished the third turn.
There is a special scenario in Monstrous Arcanum which I have never seen an event for but looks like a lot of fun and it is a lot less fussy rules wise than a full Storm of Magic game.
Righto.
If you like beautiful artwork and interesting fluff it's probably worth buying the book (though it's not cheap), but if you only want the stats for the Dread Saurian, the book is probably a waste of money.
Unfortunately
there is no other way to get stats for the Dread Saurian apart from buying the out of print book.
For all I know, since 8th edition is now out of print we MIGHT be able to put the stats and points costs for the Dread Saurian on L-O without drawing Games Workshop's ire. I am not sure.
Logically, they shouldn't be opposed because the net result is GW will sell more models, but logic doesn't always fall in Intellectual Property arguments.