>>42329718A bro is just a Pokemon that you enjoy in design/lore/gameplay and/or has carried playthroughs for you.
For example, fucking Moltres of all Pokemon is a bromon for me because every time I get it in Showdown random battles it pulls through for me with constant crits and dodges, plus it was part of a meme rain team I made for online and despite being fire type it was the MvP eclipsing even Mega Swampert.