You may notice there's a pretty clear difference between the two, just on appearance - if you look, you'll see that the average psyduck is yellow, whereas Iyaqua is blue. That's because Iyaqua is a shiny, a rare colour variant!

The current base rate of shininess is that 1 in 4,096 Pokémon will be a shiny- an improvement from the 1 in 8,192 chance of years prior.

So you may be thinking, “why on earth would you want to get shiny Pokémon?”
Well, mostly just because they're cool looking. They're not inherently better in battle, they're just coloured differently. Their inherent rarity makes them not only rewarding to find, but once you get your hands on them, they can be good for trading. But why would you want to trade away your new super-rare Pokémon?

A shiny Pokémon will entre the battlefield with a flourish of sparkles, which is why they are called “shiny”. I really don't know how this trait works biologically. Ask your local Pokémon professor or something. I'm like 10 years old.
Not every shiny colouration is cool. Some barely change, and some are... unfortunate , to say the least.

Shinies can be found encountered in the wild or hatched from eggs.