The token is named Clue Token and has the artifact subtype Clue. Clue isn't a creature type.

Discarding a card is part of the cost of Scuttletide's first ability. There is no time for players to respond to the ability before the discarded card is in the graveyard.

The card types that can appear in a graveyard are artifact, battle, creature, enchantment, instant, kindred, land, planeswalker, and sorcery. Legendary, basic, and snow are supertypes, not card types; Vedalken and Wizard are subtypes, not card types.

The triggered ability triggers when creatures are declared as attackers attacking you or a planeswalker you control. It will not trigger if an effect puts a creature onto the battlefield attacking you or a planeswalker you control.

The tokens are normal artifacts. For example, one can be sacrificed to activate the ability of Breya's Apprentice and one can be the target of Break Ties.

You can't sacrifice a Clue to activate its own ability and also to activate another ability that requires sacrificing a Clue (or any artifact) as a cost, such as that of Lonis, Cryptozoologist.