Steam Elephant
Steam Elephant is loosely used to refer to self-powered mobile steam engines. The vehicles most frequently referred to as "steam elephants" are traction engines, but railway engines have been referred to as (and occasionally named) Steam Elephant, and as a wider range of steam-powered land vehicles have emerged many have continued to be referred to as steam elephants.
The term most likely emerged from early use of steam engines in the British Army, where they replaced the use of actual elephants for heavy lifting and haulage; but its more widespread popularity may be due to the image it evokes. A traction or railway engine with a tall funnel to the fore bears a resemblance to a trumpeting elephant. A few engines have even been built to deliberately strengthen this resemblance, for the amusement of rich patrons.
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[edit source]Self-mobility seems to be a significant part of the description; it's never been popular for portable engines, which otherwise strongly resemble traction engines.
It would be hard to describe "steam elephants" as a neutral term, but intended emotions depend on the speaker; some are speaking affectionately of their mechanical beasts, while others use it with less love to refer to the bulk, smoke, noise and intrusion of engines.
At the furthest extremes of engineering, steam elephants have evolved into what might be described as landships, but practical limits are being encountered as to how large it is possible to build such things.