Glacial Crasher

A 5/5 trampler for six mana is a perfectly reasonable beater, and the card hands it to mono-blue along with a string attached: it sits frozen unless a Mountain shares the battlefield with it. That restriction is doing flavor work and color-pie work at once. Blue does not get to swing big without help, and the design encodes that by making the Crasher's aggression contingent on red's presence rather than blue's own resources. It is a deliberate dependency: a creature whose body lives in one color and whose permission to attack lives in another. The clean way to satisfy it is a blue-red shell where Mountains are already in the deck, at which point the conditional evaporates and you simply have a large trampler that blue would never be allowed to print unconditionally. Outside of red support the clause turns the card into a wall that can block but never push damage, which is precisely the cost the rate is paying for. It is a teaching card more than a constructed one, the kind of design that draws the boundary of what each color is permitted to do and then offers a two-color key to unlock it.