who can catch a splitpushing 6 items udyr with moblity boots

I hope you have hard cc.

6 item janna in some builds can outrun udyr as one I'm most personally familiar with, but bound to be others (ulting yi, singed both come to mind but I don't know the numbers).

Talon or Quinn. Question is if they want to catch him.

Hard to say, id assume a well fed assassin, fighter, or Vayne can actually kill him before he gets too far. However, a good counterplay strat would be to just have someone who can simply push harder and outduel him pushing against him. Darius is a champion who can split push and generally you'll need 2 or more to take him down. Udyr would need help and that would take players away from the next potential team fight in midlane or drag.

Don't chase him homie.

Master Yi, Vayne, Singed, hecarim, taliyah, and more.