The Beast Plain is a strange place. There are no monsters native to it, but most opponents that the party has already beaten elsewhere, or encountered and run from, or even lost to without getting a Game Over (occassionally possible) can show up here (see the Veldt section in Terii Senshi's Algorithms FAQ on GameFAQs for details). Combat gives no experience points here, but all other battle rewards still apply. This makes it a useful place to collect supplies and such without gaining levels. It's also handy for farming items from several monsters that otherwise appear only once (the Green Beret from a Satellite, to give an early example).

1-17 "Wild West" plays all over the area, even during battles. Sooner or later, after one battle or another, the boy they saw earlier suddenly appears.

They toss him some Jerky from the village. He hops over and stares at them.

This could go either way with scary/scared, I think, but Mash certainly seems to be acting like it's Gau who should be intimidated by him.

They wrestle around, while Cayenne just stands there and watches.

They wrestle around some more. Gau hops away and it takes Mash a few seconds to notice he's not wrestling anyone anymore.

Looks like the NA line was just pulled out of nowhere...

Cayenne finally decides to take action, and breaks them up.

While Gau actually imitates でござる (de gozaru), Cayenne's archaic way of saying です (desu, roughly equivalent to "is"), the English works out more cleanly when using "thou". I tried going with "art", but it just doesn't work.

Gau starts hopping around, yelling.

In the middle of this, Cayenne turns around, probably to help control his reaction.

Mash gets sick of this and pulls him off to the side.

Apparently, Mash is supposed to be explaining the situation, and doing it this way avoids a recap.

Mash and Cayenne look at each other for a few moments.

Gau is just spinning around now, and Mash notices.

Gau leaps to the bottom of the screen and makes digging motions.

The NA version is pretty badly skewed, but I have to admit it took me longer than it should have to realize what Gau's talking about. It's actually quite simple.

Mash and Cayenne are standing some distance apart. What Gau's saying is that, if Mobliz is where Mash is standing, and the riverbank where they washed up is where Cayenne's standing, then the mountain with the treasure is where he is. In other words, he's using their relative positions to map out where to go. He may be uncivilized, but he's hardly stupid...

They start walking away. Mash falls behind as the others walk offscreen.

Gau moonwalks back onto the screen.

I don't doubt Gau knows perfectly well who "Thou" is, but he's equally aware that he's getting on Mash's nerves by "mistakenly" attaching it to the wrong person, and he seems to enjoy provoking Mash.

Mash chases Gau off the screen. And then, 2-6 "Spinach Rag", which sounds like a piano piece out of the early 20th century or so, starts playing, and a green vaguely humanoid creature walks on to explain Gau's skills...

The Kappa has been imitating Gau's actions, and now looks like Gau and does an impersonation.

The Kappa changes back to itself.

The Kappa starts walking offscreen, trips and falls, gets up and waves, and finishes leaving. I guess it's supposed to be funny.

Gau's commands work as explained; see his character entry for details. One of the more useful side effects is that many Rages allow Gau to cast spells without using any MP and even without knowing the spell, though uncontrollably. Also, Gau takes on the resistances and immunities (but also weaknesses and status effects) of the monster he chooses, for as long as the Rage lasts (status effects may persist even after battle, depending on the effect). It takes time and effort to build up his skill list and learn which monsters work best, but ultimately Gau can become an incredibly effective automated powerhouse, especially once late-game equipment and monsters become available.

Gau has a brief speech that comes up when he returns from leaping: