Bramblin appears in Pokémon GO for the first time. Its debut comes with the Fall Marathon: Buddy Trek event, Tuesday, October 13, at 10:00 a.m. to Monday, October 19, 2026, at 8:00 p.m. local time.
How to encounter it
by completing the event GO Pass tasks;
by using Incense (excluding Daily Adventure Incense), throughout the event;
as a reward from some event Field Research tasks.
Niantic says you'll need a bit of luck to find it. The announcement doesn't mention a shiny Bramblin.