20" tall 15" by 16" enclosure

[Reaganomics]

Can i put a one horn chameleon in it? if not what can I?

[siz]

Well, you could, but not for long. Also, chameleons thrive in open air enclosures (mesh). I personally have a veiled chameleon in a tank but it is much, much bigger than that.
I don't know if you're into geckos at all but you could put a couple different species in there - I would recommend a crested gecko, great species to handle.