Actually, a methylene group is simply any compound which contains a C=C double bond group and the rest are single bonded carbon groups. Some example of the isomers of C6H12 which contains methylene group is:
1-hexene
2,3-dimethyl-2-butene
2,3-dimethyl-1-butene
2-methyl-2-pentene
trans-2-hexene
4-methyl-1-pentene
cis-2-hexene
trans-3-hexene
2-ethyl-1-butene
2-methyl-1-pentene
3,3-dimethyl-1-butene
4-methyl-cis-2-pentene
cis-3-methyl-2-pentene
trans-3-methyl-2-pentene