You need to understand the differences between 'passive' active abilities and just passive status.
passive active abilities are the ones that does not require triggering, but usually auto-triggers when a creature attacks, etc.
for example, deathstalker's deadly poison is a passive active ability that activates when deathstalker attacks.
passives, however, are the status that could be found when u put the mouse cursor on a creature and wait for the status window to pop up. graboid has 'burrowed' passive, devourer has 'devourer' passive, airborne creatures have 'airborne' passive, etc.
a mutant keeps the passive of the creature that they currently look like. for example, a mutant devourer, regardless of whatever ability it has, retains the devourer's original 'devourer' passive, and therefore takes one quantum from the opponent and transfer it as a darkness quantum to you.
an easy way to distinguish passive (or active) active abilities from passives is lobotomization. all passive active abilities can be loboed, while passives are never loboed. the only way to remove passive, at the moment, is to remove airborne passive by web ability.
@PS : pendulums are like normal pillars/towers and you can have more than 10 in your deck