Some animals like tigers, lions, cats, etc do this action before attacking their prey suddenly and unexpectedly. In fact they seem waiting insidiously (?) until the appropriate moment for attacking and (usually) the poor prey is not aware of their presence.
What is the verb, phrase, or idiom for describing this action/ state?
I have found "ambush" but I don't know wether it can be used for animals or not.
The usual verb is stalk which is defined by Merriam-Webster as:
to follow (an animal or person that you are hunting or trying to capture) by moving slowly and quietly.
Consider poised to pounce or poised to strike.
balanced and prepared for action
(Collins)
From Bicycling Magazine's Mountain Biking Skills, p69:
When felines are poised to pounce, they're loose, not rigid.
From Gavin Ehringer's Western Horseman's Rodeo Legends:
Like a lion poised to pounce, Whitfield sets up for a winning run at La Fiesta de los Vaqueros Rodeo in Tucson.
I think that felines pounce and snakes strike, but that's not absolute.
I would say the feline is crouching, ready to pounce. See the images of ready to pounce google gives you (I reproduce some here):
Crouch in Oxford Learner's Dictionaries:
1 to put your body close to the ground by bending your legs under you
Pounce in Oxford Learner's Dictionaries:
to move forward suddenly in order to attack or catch someone or something
The lion crouched, ready to pounce.
While stalking their prey, predators get as far down on their haunches as possible as soon as they're ready to pounce. This allows them to store energy in their tendons, that enables them to close what distance is left in a heartbeat.
Of interest may be the classic swishing of the tail, which is thought to betray their anxiety about whether or not, and at which moment, to strike.
I would suggest "lie in wait," meaning to conceal oneself, waiting to surprise, attack, or catch someone. [OxfordDictionaries.com]
Some of the images included in the question show animals stalking their prey, but the verbal description of the phrase sought by the original poster leads me to believe that that is unintentional.
Consider,
creep
: to move along with the body prone and close to the ground
The lion crept toward the antelope, moving perhaps an inch at a time. Enrichment Reading
lie at (the) catch/upon the catch
The noun “wait” once had more meanings that it does today. For example, it used to mean a watchman or guard.
Today we use it mostly to mean a period of waiting (as in “an hour’s wait”), but something of the old meaning survives in the expressions “lie in wait” (dating from around 1440) and “sit in wait” (before 1300).
In the sense of lying in ambush, English speakers once also used the phrases “lie at catch” and “lie upon the catch.”
lurk
lie in wait in a place of concealment, especially for an evil purpose.
Poised, primed, cocked, loaded, sprung, stalking maybe... A snake would be coiled.
Removing the criterion of one word: in the zone.
For the best word we must go to the Sanskrit: Ekagrata (https://en.wikipedia.org/wiki/Ekagrata)
Ekagrata is a state of single pointed natural focus. The cat is in Ekagrata. A Kalarippayattu fighter from southern India aspires to attain to Ekagrata.
But the cat is always in a state of Ekagrata! The fighter must learn from the cat!
Sometimes just prior to actually pouncing, panthers (and even people!) are described as being
“in attack mode.”
(description of such a panther from ‘Lost on the Mountain’ by Elisabeth Williams and of such a person from ‘The Taster: A Novel’ by Jonathan Hickman, both via ‘Google Books’)
Camouflage, also called cryptic coloration, is a defense or tactic that organisms use to disguise their appearance, usually to blend in with their surroundings. Organisms use camouflage to mask their location, identity, and movement. This allows prey to avoid predators, and for predators to sneak up on prey. 1
Three of the four felines in the images posted by the OP are sneaking up on their prey so as not to be seen. (Google images)
sneak up on
To approach a person or animal without being seen or heard
Camouflaged against the tall grasses, cheetahs quietly sneak up on their prey until they are confident about the attack, burst out using their tail as a rudder, trip the animal with their paw, and then suffocate it with a bite to the neck. After making the kill, cheetahs must eat quickly or drag the food to a hiding spot before any lions, leopards, or hyenas steal it.
