There has also been some additional "rationales" for why Howard the duck.
I do believe MovieBob has the "most likely" reason.
But here are some additional reasons that seem to be creeping around the geek-verse. I'll post each one independently as I come across them:
Howard the duck is a shout out to both the possibility and the closing of several chapters of legal fights between Disney and both Marvel and Lucasfilm. Sparing over property rights and what is fair use is nothing new in Hollywood. Oddly, all three companies got in a scrap over Howard the Duck. When Howard first appeared in the 70's, he was a part of the edgier use of anthropomorphic characters in using adult themes. This clashed with Disney more "sanitized" use in the 1970. A legal fight broke out over how Howard kinda remotely looked like Donald. This was mostly a nuisance lawsuit and Marvel quickly won this. However, as MovieBob pointed out, Howard brought under more legal challenges with creator when Lucas wanted to buy rights to make full movie version. Disney jumped in again, but since the mouse had made no friends on either side of the case, they were again dismissed.
This trailer now is kinda a fun shout out "We own them all now, Bithes!" Not so much in a mean spirited way, but a certain acknowledgement of the weird story how Disney, Marvel, and Lucasfilm are, after some very bizarre set of cultural and business circumstances, now way all under the same tent.