What is this symbol next to the Pokémon's markings?

Solution 1:

That would be the Origin Marking.

From generation 6 onwards, a marker is placed next to any Pokemon that was raised in that generation.

The specific types of known origin markings thus far are:

• The blue pentagon: Used for the generation 6 games. Specifically, this is used for Pokemon from X and Y and Omega Ruby and Alpha Sapphire.
• The black clover: Used for the generation 7 games. Specifically, this is used for Pokemon from Sun and Moon, as well as Ultra Sun and Ultra Moon.
• The Gameboy icon: Used for Pokemon obtained through use of the Poke Transporter from the 3DS virtual console. This includes all remakes, for both generation 1 and 2.
• The GO icon: Used for Pokemon obtained from Pokemon GO. Only used in Let's Go Pikachu and Eevee so far.
• No origin marking: Used for Pokemon obtained in generation 3, 4 and 5, as these do not have origin markings.

In this case, the icon is the black clover, meaning the Pokemon was obtained in a generation 7 game.