These are “monsters” that you can ride.
deets here. https://github.com/CleverRaven/Cataclysm-DDA/pull/32409
Three varieties : combat mech, recon mech, lifter mech.
Find them in lab finales and military bunkers, bring an ID card or a control laptop.
And sometimes their propietary batteries spawn in similar places too.
Have fun bashing stuff and moving cars out the way and firing their lasers.