What function does で have in the sentence 馬は動物である?


I have a denshijisho, always giving me helpful expressions attached to a
kanji/word etc. and this time it was

> 馬は動物である。  
>  A horse is an animal.

in which I don't understand the function of the particle de.




「で」 in this sentence is a part of 「である」. 「である」 means 「です」 in ~~respectful
speech~~ or written speech.

