Pegasus Guardian

If a token is exiled this way, it will cease to exist and won't return to the battlefield.

If a spell is cast as an Adventure, its controller exiles it instead of putting it into its owner’s graveyard as it resolves. For as long as it remains exiled, that player may cast it as a permanent spell. If an Adventure spell leaves the stack in any way other than resolving (most likely by being countered or by failing to resolve because its targets have all become illegal), that card won’t be exiled and the spell’s controller won’t be able to cast it as a permanent later.

If you cast an adventurer card as an Adventure, use only its alternative characteristics to determine whether it’s legal to cast that spell.