Well, that goes back a long way, and it really depends on how you define a tank.

I mean, you go back far enough, a chariot could be considered mobile shock power on wheels, though I believe most folks don't go far beyond, I think it's the 15th Century, with the Hussite War Wagon.

Not really a weapon of maneuver, as the motive power was kindof vulnerable, but it did armor the folks inside shooting out.

Move onwards a century or two, and one gets to Leonardo da Vinci's design. Human-powered, but mobile, armored, and equipped with cannon. It's often considered the first tank design.

Then things get a little hazy.

Probably the first out the box for what we would consider a tank today was a Frenchman called Levavasseur, who in 1903 came up with an armored box with an internal combustion engine with a 75mm gun up front which ran on tracks. His proposal did not meet with approval from the French higher echelons, and it was never built.

Shortly thereafter, HG Wells writes "The Land Ironclads". He uses pedirail wheels instead of tracks, but the general gist of it is that the thing is a tank.

Two more proposals showed up in 1911. One from an Aussie, named Lancelot de Mole, and the other an Austrie named Gunther Burstyn. The former looked a fair bit like a squished WW1 tank (Or maybe a Medium D / M1921) the latter, to all intents and purposes, was a modern tank design with tracks, internal combustion, and a rotating turret for the armament. Again, neither project was taken up.

That was basically it until WW1.