...what would they have?
I'm putting this here rather than in anime because there's non-trainers in the games, the comics, and other sources as well. If someone's a trainer in one series but not in another, heck, put them down here anyway!
Domino would have a Roseraid, especially if she could find one with mutated roses that looked like tulips.
Jirarudan would have to have a flying type, and if it *had* to be something that wasn't Legendary, I could see him with a Pidgeot. Although he sure as hell wouldn't raise it from a Pidgey, and he'd probably insist on a pedigree.
Scott would have a Drifloon AND YOU ALL KNOW WHY.
I fully agree with Shedinja that Sebastian would have a Porygon Z.
Madame Boss...a Delcatty?
I can see Miyamoto with a Crobat, but I'm not sure why.
Grandpa from Magical Pokemon Journey (the Natu's not his, it's Ginger's, so he still counts) would have a Slowpoke. It would share his passion for mad science but would never be able to accomplish anything.