You can cast an instant spell whenever you have priority on any player's turn
During its release period between March and September 2007, the Umbreon Star was made widely available for the first time
But the only way to differentiate them at that point is to make really complicated logic, so you constantly need to make more complicated stuff. Creatures knows Pokmon must, at some level, be something any child can learn quickly, so its opted to bump up the numbers instead of introducing too many new ideas
On modern English cards it sits in the bottom-left corner, next to the collector number, for example 25/198
Ravensburger Pokmon Labyrinth: Navigate the Maze of Pokmon Treasures
There is some novelty in being able to convert any kind of counter into +1/+1 counters (or flying counters), but novelty is mostly all it offers for six mana