This is the 5951st most frequent Polish word.
zwierzęciem
"Zwierzęciem" means "animal" (instrumental case) in English.
In this sentence, 'zwierzęciem' is in the instrumental case, and it indicates the object being taken care of.
On opiekuje się rannym zwierzęciem.
He is taking care of an injured animal.
Here, 'zwierzęciem' is in the instrumental case used after the preposition 'o' to describe the topic of the conversation.
Rozmawialiśmy o tym zwierzęciem.
We talked about this animal.
In this context, 'zwierzęciem' serves as the instrumental case of 'zwierzę', used here within a relative clause to describe the observed entity.