does tapping a creature with an ability work as an instant on the stack?

does tapping a creature with an ability work same as an instant on the stack?

...what?

is creature ability an instant?

Not at all. A creature ability is a creature ability, not an Instant.

Something is only an instant on the stack if one of its card types is "Instant," or if it specifically says "considered an Instant." I can tell you with full confidence that no creature ability printed thus far says anything of the sort.

I meant is it same speed as instant on the stack

Ah, okay. Well, I'll first say that there's no such thing as "spell speed" in Magic (unlike a game such as Yugioh, which has Spell Speed 1, 2, and 3).

However, a creature's activated ability has the same timing restrictions as Instants do -- unless stated otherwise. Brain Weevil, for example, says "activate only any time you could activate a sorcery." This doesn't mean it counts as a sorcery. Rather, it means you can only activate its ability at times you would be able to activate a sorcery, which is while the stack is empty and you have priority.

So, creatures like Prodigal Sorcerer or Arcanis the Omnipotent can activate their activated abilities anytime you have priority, which is the same timing restriction as Instants have.