Tall animals, for example, a elephant weighs 100kg at least.
Or, should I only use the plural of the word elephant?
I would use the following:
Tall animals, such as the elephant, weigh at least 100 kg.
When we speak of an animal as a type in a formal or scientific context, it's common to say "the elephant", "the zebra", etc.
If you want more understanding of article use, try this link.
The information which follows is not absolutely important to understand thoroughly, but I'm including it in case you want to know more.
I prefer using the "such as" form, rather than "for example", because "such as" is smoother inside a sentence. You can use "for example", but it's usually either at the beginning of a phrase or it feels more like a break in the sentence.
Here are a couple examples of how "for example" might be used naturally:
Tall animals (for example, the elephant) weigh at least 100 kg.
Tall animals -- for example, the elephant -- weigh at least 100 kg.
The following is probably acceptable to most, but it feels unnatural to me. (I'm a reasonably literate American native speaker.)
Tall animals, for example, the elephant, weight at least 100 kg.
(Yes, I realize that in this day and age there is a bit of irony in the phrase "reasonably literate American", but you get the idea.)
The reason it feels unnatural is that there's too much of a break of the main idea, so it's a parenthesis which feels better with punctuation marks indicating more of a break from the main idea.
It should read:
"Tall animals, for example, an elephant, weigh 100kg at least."
Everything else is fine. The plural is unnecessary. You have to make the subject and verb agree. "Tall animals" is the subject and "weigh" is its verb.