In the second Night Elf mission in the campaign mode (Chapter 2: Daughters of the Moon) you can find a Hydralisk at the top of the map, slightly to the left of the top right fountain of life. The easiest way to find it is to create an owl scout and fly it up towards the fountain and then move left along the top of the map until you see it. I don't think it is possible to fight it, but it is a nice easter egg for Starcraft fans.