Jurass-is so fat it has a larger gravitational pull than other masses but it still doesn't off set the regular orbit of the planets that we currently have because actually everything has a slight gravitational pull...ohhhhh
I couldn't resist. I'm not sorry.
ANYWAYI thought I would bring up some of the animals that were about in a traditional "prehistoric" period of time. Some of my favorites anyway. I shall include links to the wiki page in the names if you want to go that extra layer of dip deeper.
AepycamelusThese babies are essentially the mash up of a camel and a giraffe. They were found in Colorado, Kansas, North Texas ect. They were prairie dwellers.
AmebelodonThese guys are weird
They are obvious pre elephants. They were found around the gulf coast and great plains areas.
AndrewsarchusThese guys are carnivores. They were probably proliferate in California.
Borophagus (Bone-crushing Dog)A canine. Not directly related to the wolf, but close.
There are a lot more.Not all are giant, there were a lot of pygmy types of animals. Pygmy elephants and what have you.
There were also a lot of animals similar to what we have now. Bears, Bison, Warthogs and others.
So basically if in doubt, use an animal we already have but make it larger than normal or smaller than normal.
Have fun with it.
I think Ashling is going to have a bone crushing dog, just throwing that out there.