Non-uniformly shaped cake methods
A lot of those nicely shaped cakes are made from a rectangular or round cake. You just cut the required sizes and shapes so you end up with something T-rex looking. You put a bit of frosting between each pair of pieces, so that they stick together and the cake does not fall apart. Usually the whole thing is covered with fondant, so you cannot see the separate pieces.

This is a youtube example using this method for a dinosaur cake.

Using a mold is also a possibility, but very likely that you'd end up with pieces of cake that are over- or undercooked, because you have smaller and larger parts in your dinosaur. You should also be very confident that the cake would slide easily out of the mold, because an odd shape is more fragile than a rectangle or a circle.