The Grass Snake (Natrix natrix) is a Eurasian non-venomous snake.
It is dark green, brown or grey in colour with a characteristic yellow collar behind the head, which explains the alternative name Ringed Snake.
The underside is whitish with irregular blocks of black.
Grass Snakes are strong swimmers and are often found near fresh water.
They feeds almost exclusively on amphibians.
Not being venomous, the snake's main defence is to produce a garlic-smelling fluid from the anal glands, or to feign death (thanatosis) by becoming completely limp.
They may also perform an aggressive display in defence, hissing and striking without opening the mouth.
They rarely bite in defence.
When caught they often regurgitate the contents of their stomachs.

I found this very unfortunate Marsh Frog (Pelophylax ridibundus) in the mouth of a Grass Snake!
It was a sad sight to see but Mother Nature is brutal.
The snake held the frog in its mouth for at least 1,5 hours before it could swallow it.
Until the end I could see the frog breathing and the look in its eyes somehow showed a desperate hope that the snake would get tired and release it from its mouth.

Hortobágy-Halastó, Hortobágy National Park, Hungary, 2 May 2019