What it does
"Synapse Creature" isn't a standard Magic keyword or ability—it doesn't appear in the comprehensive rules. You may be thinking of a creature type, a flavor term, or possibly confusing it with something from another game (the Tyranids in Warhammer 40K use "Synapse Creatures").
In Magic, the closest real mechanics involve creatures that buff or control other creatures, like those with "Lord" effects, or the recent Universes Beyond Warhammer 40K Commander decks, which did include Tyranid-themed cards. If you saw "Synapse" on a card, it might be specific card text rather than an evergreen keyword.
If you can tell me the exact card name or where you encountered the term, I can explain the actual rules and how it plays in Commander—including any nuances and timing interactions that commonly trip players up.
Iconic cards with Synapse Creature
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