Maybe during the time they were on the run from the guild and Jaihi was doing his quests?
That might be the case ^^
Maybe, Dumbo doesn't like anyone else and makes it really noticeable like Dumbo like them riding on its back or doesn't try to help them.
But we see with Jaihi and Nine on the run that Dumbo does have some favoritism towards Jaihi and we did just witness Dumbo rescuing Jaihi.
I wonder if Dumbo is actually a sort of familiar for Nine and thus those who Nine likes/doesn't like Dumbo likes/doesn't like?
Nine: Dumbo has taken a liking to you
Me: When did that happen?