The most awesome mutation ive seen was graboid with hatch that turned into a devourer (1/7) that retained the burrowed status and had 1
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cost destroy permanent.
In other words it stole 1
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and thus paid its own ability for free while burrowed and immune to targeting spells.
The other one i liked the most have been a graboid with burrow. The only way to make a graboid unburrowed.