Can Beast Boy turn into a hidden animal, like Bigfoot, Nessie, or Chupacabra?
For a single episode in the Teen Titans cartoon, he was able to turn into a Sasquatch:
It's pretty often that we see him transforming into dinosaurs (examples: Teen Titans cartoon, Teen Titans GO cartoon). This power dates back very far and was mentioned in this 1982 interview with the co-creator of Teen Titans. Although we don't see him transform into a plesiosaur (which is essentially what the Loch Ness Monster is supposed to be), there's no reason I see he couldn't.
In the comic Teen Titans Go! #36, he transforms into a Gordanian, a type of alien:
In this comic Robin says that "Animals are [Beast Boy's] thing, but he's taken on other forms... Not without a lot of effort".
Beast Boy has the ability to morph and transform into any animal that he has seen himself or has seen in an illustration (as is the case when he shapeshifts into an animal such as an extinct dinosaur).
The given citation is Wallace, Dan (2008), "Beast Boy", in Dougall, Alastair, The DC Comics Encyclopedia, London: Dorling Kindersley, p. 39, ISBN 978-0-7566-4119-1