The Immortal Snail

When this creature enters the battlefield exile
it with {x} travel counters on it. When there are
no travel counters return it to the battlefield. If {x} is greater than four this creature gains
trample, if {x} is greater than ten it also gains indestructible,
if {x} is greater than fifteen it also gains shroud.
The Immortal Snail's power and toughness are
equal to its converted mana cost.