Ugh. I hate nightmare phases. Seems no matter how hard you try to shelter them from "scary stuff", preschoolers *always* hit a phase where they wake up screaming about the monster under the bed, the spider in their pillow, the lady screaming, the dog in the closet.
A trick that worked well for us was to make monster banishment part of the night time ritual. After we read story and gave out kisses, I'd turn to the room, clap my hands a couple times and say, "OK, monsters. It is time for bed now, so you have to go home! You can come back to play tomorrow after breakfast!"
Weird as it sounds, it worked like a charm on my preschoolers. Perhaps because Mommy is the Ultimate Source of What Will or Will Not Happen Around Here? After all, when I say it's time for their "real" friend to go home, s/he goes home! So it follows, if I tell the monsters they need to go home, of COURSE they just...do it?
Don't really care why, of course - I'm just grateful that it *works*! ;-)
Familiarizing him with lizards per the other suggestions may help, too. Something about them is probably creeping him out, fascinating by day and horrifying at night. That was what we had with our five year old daughter - all day long, she'd beg for a puppy. And then at night, she'd have nightmares about a "mean" dog in the closet or window. **sigh**
Don't make yourself TOO crazy looking for a logical reason for it, though. There might not be one - preschoolers' minds are not fully-formed logical thinking machines yet. I think that may be why they often respond better to the "monsters, go home now" routines than the carefully thought out lectures: "Now honey, blah blah imagination yadda yadda not real something about 'when I was your age' whatever-whatever-whatever blah-blah-blah".
Hang in there! Those wee-hour wakeup calls (screams?) are absolutely draining - make sure you take care of YOU too!