The plural of हाथी is हाथी itself because it is a masculine noun. However, in the oblique case (when it is followed by a postposition), the plural हाथी becomes हाथियों while the singular हाथी remains the same.

You can write 'I saw elephants' in two ways:
मैंने हाथी देखे । - where 'हाथी' is in the direct case and the form of the verb देखे tells you it is plural
मैंने हाथियों को देखा- where हाथियों is in the oblique case because of the postposition को

You can think of the two forms as being like 'see' and 'look at' respectively. However, keep in mind that when talking about seeing humans or pets, you use the second form ( को देखा) exclusively.