Changelings!!

 

 

 

There isn't much info to be had on this species/race.  Of its members are king Kold and Frieza.  They are changelings because they have many different stages which they transform into to gain their full strength.  Like Frieza, they can not use or sustain heir full strength unless they transform into a higher form, which require a lot of energy.  They already have the energy to transform into their next level, and do not need to become stronger to be able to do so.

Its easy to classify other characters in DBZ as changelings, Oolong, for example, can 'morph' into any shape, animal or person, as can Puar.  They are not changelings because which ever form they take, their strength remains the same (although, in Dragonball,  Oolong took on the form of a ferocious beast, he did a good job [for a while at least] of hiding his strength (and being Oolong, that isn't very much).  It is the same with Puar.

Captain Gynu is another who is easily confused with a changeling.  He can move from his own body to that of another, so he does not change into a different stage of his own, and although he can tap into the powers and skills of the person whose body he has 'borrowed', he can not use its full potential, at least not with out a LOT of practice.  We can't be sure if the body we see Captain Gynu in is his original, or whiter he just came across it and liked it. 

Saiyans can transform into their Oozaru form, which also gives them a massive increase in strength.  They are only able to do this if they have the monkey  like tail they are born with.  If they loose their tails, they can not transform.  They do not need a certain amount of energy or power to become Oozaru, just a tail and a full moon.  Saiyans can ascend to different levels, SSJ1, SSJ2 (you know how it goes).  Saiyans need a certain amount of power and emotion before they can transform, but it does become spontaneous with time.  Like changelings, Saiyans gain a great increase in power when they transform