This is the 18486th most frequent Russian word.
ядовитым
"Ядовитым" means "poisonous" in English.
Here, 'ядовитым' is used as an adjective in the instrumental case to describe the characteristics of the noun 'существом' (creature).
Змея оказалась ядовитым существом.
The snake turned out to be a venomous creature.
In this sentence, 'ядовитым' modifies 'растение' (plant) and is used in the instrumental case to indicate its appearance.
Растение выглядит ядовитым, поэтому его не трогают.
The plant looks poisonous, so nobody touches it.
'Ядовитым' in this sentence is metaphorically used to describe the nature of the remark ('замечанием'), signifying something harmful or biting.