@johnnyblues I can tell you that I imported one in December. (feet and tusks) I did not pay for or complete any paperwork above and beyond what was required for the other animals such as kudu, impala, bushbuck, duiker, cape buffalo, waterbuck. I did not pay more to Coppersmith for import on the Hippo's account. The export fees were the same. The dip and pack fees were the same.
So, while I don't know "what is required" I know that no special costs or time were required to get the hippo delivered to the USA.