When you are talking about people or something with its own agency, it is mandatory to use को (or use the dative form if it is a pronoun). Eg: उसने मुझे देखा(He saw me), मैंने भगवान को देखा (I saw God), जूलिया ने पीटर को देखा (Julia saw Peter) etc. This is also seen when you are asking questions. You say तुमने किसको देखा? (Who did you see?) instead of तुमने क्या देखा? (What did you see?)

For everything else, you can either use को or leave it out (in which case the noun becomes a direct object of the verb देखना). So, both मैने हाथी देखे and मैंने हाथियों को देखा are correct. [Notice that in the first sentence, देखना takes the gender and number of हाथी because it is its direct object. In the second sentence, देखना has no direct objects and so is seen in its default singular, masculine form]

The form with the को may be used to distinguish 'look' from 'see'. Also, the को form is often used with pets and other animals when you're not talking about them impersonally.