My espeon smacks his tail on the ground repeatedly when he’s upset. Why does he does this

It’s actually common body language, anonymous. When a pokémon like your espeon thumps its tail against the floor or ground, it’s expending pent-up energy. In some cases, this can be a positive thing—an outlet for excess happiness—as in the case of dog-like pokémon such as growlithe. However, among cat-like pokémon such as espeon, it’s another way of emphasizing his annoyance with you (or a situation), as he’s expending pent-up irritation. Think of it like letting off steam by doing something productive or by screaming into a pillow or so forth.