Morph is a floating gel-like creature which can shape into any form (but not change size) as well as imitating voices to some extent. It appears to be non-sapient (though read on) but intelligent, and is a very docile creature. It's natural state is that of a pink gel with a mouth and two eyes; which is able to float around and keep bubble-like portions of itself near its body but not touching it; by some unknown means.
The infamous cyborg pirate Long John Silver rescued a Morph on Proteus and kept it as a loyal pet. His remark when introducing it: "he's a Morph" implies that this is the name of the species, rather than that individual. After arrival on the legendary Treasure Planet; Silver leads a mutiny to take control of the ship and Morph accidentally plays up an important role when he playfully disguises himself as the treasure map and is taken by Jim Hawkins; causing Silver and the pirates to believe that Hawkins has the map. After witnessing his owner's violent and arrogant behavior, Morph changes side and becomes one of Hawkins companions. He eventually forgives his redeemed ex-owner Silver, who gives Morph to Hawkins and departs alone for new adventures.
These actions suggest some level of advanced comprehension from Morph and might imply that his species is semi-sapient.