Trickster's Elk overwrites all previous effects that set the creature's base power and toughness to specific values. Any power- or toughness-setting effects that start to apply afterward will overwrite this effect.

Auras attached to a creature don't become tapped when the creature becomes tapped. Except in some rare cases, an Aura with bestow remains untapped when it becomes unattached and becomes a creature.

Trickster's Elk overwrites all colors and creature types the enchanted creature has. It's just a green Elk. The creature keeps any supertypes (such as legendary) it has but loses any other card types it has (such as artifact).

Unlike other Auras, an Aura with bestow isn't put into its owner's graveyard if it becomes unattached. Rather, the effect making it an Aura ends, it loses enchant creature, and it remains on the battlefield as an enchantment creature. It can attack (and its  abilities can be activated, if it has any) on the turn it becomes unattached if it's been under your control continuously, even as an Aura, since your most recent turn began.

On the stack, a spell with bestow is either a creature spell or an Aura spell. It's never both, although it's an enchantment spell in either case.

If a permanent with bestow enters the battlefield by any method other than being cast, it will be an enchantment creature. You can't choose to pay the bestow cost and have it become an Aura.

Trickster's Elk may enchant a permanent that is only temporarily a creature, such as a Vehicle. If this happens, Trickster's Elk's effect causes the enchanted permanent to remain a 3/3 green Elk creature even after the temporary effect making it a creature expires.

If the enchanted creature gains an ability after Trickster's Elk becomes attached to it, it will keep that ability.

If the enchanted creature has any subtypes other than creature types, such as Equipment, Vehicle, or Urza's, it loses those as well.

Because damage remains marked on a creature until the damage is removed as the turn ends, nonlethal damage dealt to a creature may become lethal if Trickster's Elk becomes attached to it during that turn or becomes unattached later in the turn.

Unlike other Aura spells, an Aura spell with bestow isn't countered if its target is illegal as it begins to resolve. Rather, the effect making it an Aura spell ends, it loses enchant creature, it returns to being an enchantment creature spell, and it resolves and enters the battlefield as an enchantment creature.

Effects that modify the creature's power and/or toughness, such as the effect of Wing It, will apply to the creature no matter when they started to take effect. The same is true for counters that change its power and/or toughness and effects that switch its power and toughness.