Let's invent some very weird pieces

It could be a piece stronger than the Queen in the opening and middle game.

If the Hawk is Seirawan's Hawk or Musketeer Chess Hawk especially the piece is a hell on the board.

Yes, I like the hawk from Musketeer chess (jumps 2 or 3 in queen's directions).

It is used in infinite chess in the Jäger units, and starts in a position which protects the rearmost pawns:

Chess only has one leaping piece (the knight), so the hawk is a great addition because of its simple pattern.

A king+hawk is starting to get pretty powerful - They don't duplicate each other so it would be a piece that attacks 24 squares. (To me - it deserves a name more powerful than "kangaroo").

the spooky knight

moves 2 knight moves in one move. it can't capture on the first move and instead walk through it.

(I meant walk through another piece)

Awesome! Maybe too strong ... is the 2nd move optional? Can you choose to make only one Knight's move?

um... what do you mean "optional"? does it means "normal knight move?"

evert823 meant if the spooky knight can move as a knight or not. For example, can the spooky knight(represented by the plain knight) take the black pawns, or can it move to any square with a piece on it?

nope

but if the rooks are black, it must capture it if it moves to there.

first move can go into the pieces, second move normal move.

I found a very interesting piece, it is a fusion of orphan (piece that moves like opposing pieces that attack him) and friend (piece that moves like friendly pieces that protect him), called gamin.

Well, not only a fusion, it has special extra properties:

Gamin piece (original fount)

What about Sheldon Copper's chess pieces?
I like the Prince Joey: "The King's feeble-minded but well-meaning cousin. Every time he moves, there's a one-in-five chance he'll kill himself."

I would raise the idea of this piece to be like a kind of "superqueen" or "amazon", maybe with a 1/6 chance of die, instead of 1/5 (for be able of use a common dice).

See the Hook Mover in large Shogi variants

The hook mover does not move twice, it simply can change direction once per move. Onesi's pantheon would be able to double capture, unlike the hook mover.

That is interesting.

Is there some 3D chess variant with this piece?

I am confused now. Are Onesi and Angel3D the same person? I only find the Pantheon described in a posting by Angel3D. But that states that the Pantheon is not a hook mover at all, but an 'area mover', meaning that it can only move where the rectangle having its start and target location as corners is completely empty. And it certainly cannot do any double capture.

The piece described as hook mover, in another posting, was the Seraph. It moves diagoally, though, and double capture is explicitly denied for this piece.

Royal ghost (inspired in clash royale):

The royal ghost would be a piece that moves like a king, guardian or mann (one step to any adjacent square).

However it isn't royal (in the sense that it doesn't suffer checkmate) unlike king, being more similar to the guardian piece.

And unlike the guardian piece (and any other piece) this piece is invisible to the opponent.

The opponent only can detect or see this piece, when this piece captures one of his own, or at its initial position or entry in the board.

After being moved in other turn, the royal ghost becomes invisible to the opponent again.

After move the royal ghost (mentally, since can't be represented in the board, it is invisible to the opponent) the player must declare said movement. "I moved the royal ghost".

Since the opponent can't see the piece nor know where is, and he is unable to know when the other player is doing traps with the location of this piece, it is important to have a judge in the game that knows where the royal ghost of each player was moved in each movement...

or simply do that each player write in a paper where he has moved his real ghost and after end the game, see that none had cheated.

Since your royal ghost is invisible to your opponent, and vice versa, it is possible to have more than one royal ghost (your royal ghost and the opponent royal ghost) or have a invisible royal ghost and a opponent piece (opponent moved said piece after player moved his royal ghost) in the same square.

This would be quite tricky to play in an otb game, also difficult here on forums, but I think it would be quite fun.