Mairsil, the Pretender

If Mairsil has an equip ability, activating it won't cause anything to happen. Mairsil doesn't become attached to a creature. They may remain friends.

If you have a creature enter the battlefield as a copy of Mairsil, its first ability triggers. You won't be able to activate any abilities before the "legend rule" applies, but the triggered ability will still let you exile another card with a cage counter regardless of which Mairsil you keep.

If Mairsil gains an activated ability that's normally linked to another activated ability of the card it came from, those two abilities Mairsil gains are linked for as long as Mairsil remains on the battlefield. For example, if Mairsil exiles Izzet Chemister, cards exiled with the first ability Mairsil gained from Izzet Chemister can be cast if you activate the second ability it gained that way. Izzet Chemister itself can't be cast this way, and if Mairsil leaves the battlefield before activating that second ability, the cards exiled with that first ability are lost forever.

If multiple cards exiled with cage counters on them have the same ability, Mairsil will have multiple instances of that ability. Each may be activated once each turn.

If another player gains control of Mairsil, it will have the abilities of only cards that player owns in exile with cage counters on them.

If Mairsil gains an activated ability that's normally linked to a non-activated ability of the card it came from, the ability Mairsil has isn't linked to any ability. For example, if Mairsil exiles Prototype Portal, the activated ability creates no tokens at all; it doesn't create Prototype Portal tokens.

If one of Mairsil's abilities doesn't resolve, most likely because its target became illegal before it resolved, it can't be activated again in the same turn.

If Mairsil has a crew ability, creatures can crew Mairsil. It'll become an artifact creature, but its power and toughness remain unchanged.