Since these big hitters will likely be the highest HP Pokmon cards in the meta for a while, giving your opponent at least one swipe at them before you get to attack is a fair balancing mechanic that returns from the previous iteration of Mega Evolutions in 2016's XY Pokemon sets
These can be a specific type of ability the monster has (Flip, Spirit, Union, Gemini, Tuner, Pendulum), or indicate that the card needs to be Special Summoned in a specific way (Ritual, Fusion, Synchro, Xyz, Link)
This is for ONE (1) Chinese Surprise 151 Booster Pack
The VMAX tag, the Pokemon TCG's version of Dynamaxing and Gigantamaxing, makes this incredible scene possible, as Umbreon reaches a mammoth size
Towards the end I started to run low on time so, sorry again if it got messy