Yes, they have. Kyogre and Groudon --> Rayquaza. A whale, a giant land monster-thing, and a sky dragon. Palkia and Dialga --> Giratina. Weird pink dinosaur, a large steel horse, and a centipede-like lizard (if I have to explain the centipede part one more fucking time I'll pull my hair out).Haven't the third wheel legendaries always been dramatically different from the initial two?
Also that picture is obviously fake; it's just some really bad fanart.
Also, the picture is obviously not real, but it doesn't say it is real. It's a drawing of what someone saw.