Everyone saying the Diplo needs a tail whip attack and platform saddle, seriously if you want that get a bronto, the Diplo has its purpose and so does the bronto, making one into the other makes one redundant and totally useless. Brontos are attackers and portable bases, diplos are a hybrid of speed and capacity, no one would complain about a gallimimus that can carry eleven people and still have space to spare for three tonnes of cargo, but they do complain about a Diplo which is essentially that in terms of functionality.
Not as useless as you might think! A high-level spino was terrorizing a nearby river. I took my diplo (Dylan) and while the spino was focused on killing Dylan, I was using my crossbow and rifle to do damage to it. Spino ended up dead, while Dylan was at 2000 out of 3000 health, and I was not injured at all.