This here is... probably not what you were expecting MegaKasterious to look. Yep, it's just a tanuki Pokemon who wields Kasterious, from the idea that tanukis are often accompanied by umbrellas. Male Kasterious take a Tanucast as their master, while females harbor a Sentret master. Now, realistically, Tanucast is more powerful than Sentret (even with the weather nerf) but both forms have the same stats to be fair.

Kasterious is a Psychic/Flying type with good speed and some killer Special Attack, as well as decent regular attack, but is a bit on the brittle side. Mega Form greatly raises their defenses and other stats, and come with the ability Ronin, which doubles attack and defense stats, but halves speed until it's down to half its HP. It's sort of like Defeatist, except not as crappy. It can take some abuse, but watch out. Lower speed means it may attack second, and likely won't be able to make use of its raised stats. Kasterious gets some decent support moves, though, as well as access to Sucker Punch, so it can work around its speed disadvantage.

Sentret belongs to Nintendo and Gamefreak