Dog is Alyx's robot bodyguard and playmate. Dog was first found in the Black Mesa East chapter. Alyx Vance
's dad made Dog to pertect Alyx against the aliens. Dog is a strong fighter, you will see him throw a van at a group of Combine Soldiers and destroy a Combine APC
at a building, even pull of a brian out of a Strider
! Dog also saved Alyx against a Combine Advisor
Dog has a ball that looks like a Rollermine
and a face from a destroyed Scanner Type 1
, with 3 plates instead of 4 and repainted.
Headcrabs are from Half-Life, but this is a Half-Life 2 Headcrab. Headcrabs are 4 leged creatures that hop on Citizen's head and turn them into a Zombie. Headcrabs had some upgrades, they can barry themselfs in sand or dirt and pop out and hop on your head (really for you they just scratch), the Combine had taken Headcrabs from Xen and sent them on rockets and launch them in non-combine areas such as the small town of Ravenholm.
To destroy a Headcrab, just use your crowbar, it's the easiest way. If you don't have the crowbar when you fight a Headcrab (like in HL2: Episode One), use a pistol, SMG, or shotgun. Headcrabs can be tricky to kill, also 3 shots from your Gravity Gun can kill one in Half-Life 2.