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.
Hey, got lucky and found a Combat Mech, cool addition to the game imo thanks!. Just wondering if there is anyway I can load it/attach it to my vehicle so that I don’t have to leave it behind?
considering they’re “monsters” you could probably capture them I think and put them a livestock or robot pen
I’m just imagining an Atlas stuffed in a little cattle car.
Kind of sad they don’t seem to have a vehicle-esque modification system built in, even with preset tiles and each one having it’s own restrictions on what can go in the layers having custom mechs would be nice.
but i can also see how that would be too much work for the coders and a bit out of the scope of what survivors should be capable of.
For modifiable mechs, we have a perfectly good vehicle system we can use for that potentially.
The line gets blurred when it comes to what should count as a vehicle, adn what should count as a rideable “monster” , and the line is perhaps down to how it moves.
THe current mechs are exoskeleton suits basically, and walk like humans do, hence the vehicle movement system would be a bit weird with those kinda suits - accelerating and braking dosnt fit.
If its possible to maybe bypass the vehicle movement system, but “mount” a “vehicle” instead and walk around with it, then I could rip a lot of the mech riding code out, and replace it with a vehicle… that way it can be repaired and modified, and itd fit in with existing cata code a lot neater.
If tank controls ever get added maybe you could use those? Tanks kind of handle weird at the moment since they should be able to turn on the spot, that seems like a system that could work for mechs (and ridable animals I guess)
Edit:
I mean I don’t know how much the mech suits are supposed to weigh but going from 0 to top speed instantly seems a bit off, same with changing direction when moving, wouldn’t the weight effect momentum?
Is it possible to transport these? I’ve just found the scout one and I REALLY want to take it with me, do they fit in livestock carriers? 
Edit: OMG! Yes, yes you can! muhahahaha!
Hmmmmm, would Mechs be rad proof?
I don’t believe so. You could just wear clothing/armor that is radiation proof while you ride one.
Unless your mutated.
Which brings up another question, any restrictions when it comes to using a mech?