Is this a possible structural isomer for Butene(C4H8)
Yes, the formula you drew is an example of isobutylene.  Isobutylene has the "official" IUPAC name of 2-methylpropene, and is an important industrial chemical.  Because it:


has a formula of $\ce{C4H8}$, and thus contains four carbon atoms;
contains a carbon-carbon double bond;
and doesn't contain any other functional groups


... it is indeed an isomer of butene (sometimes known as butylene).  You got this one right!$%edit$