Spawn was David's pet cobra snake who slept under David's bed. Marco morphs him in order to hide from being discovered by David's father, who is looking for an intruder after the house's alarm goes off. According to Marco, Spawn has been de-poisoned "or whatever they call it" but Marco doesn't trust that information, given the type of pet David has. However, after he is bitten by Spawn he learns that this information is correct.
When Visser Three and several Hork-Bajir storm David's house, a scuffle ensues, with Ax in his Andalite form and Marco morphed as Spawn, they realize that one of the cobras is an "Andalite bandit" in morph, but they take the real Spawn, not Marco. A Hork-Bajir wraps him up in a towel and carries him down the stairs. Marco doesn't know what Visser Three does with him.