Demon's Souls

I see these phantom frogs, what are they, can I kill it?

I see this "thing" scuttling across the floor in the tower before the second boss (that giant knight) what is it, and can I kill.

Can you lock on to them?

They're Crystal Gekkos and they have some sweet loot, the loot changes depending on the WT, the closer to black the better the loot. When one is nearby you'll hear a jingling sound but if they see you they'll start to run away and vanish shortly after. If you fail to kill one before it vanishes it wont show up again until you kill a boss in that world(yes, that means you only get a few chances at them per playthrough). If you can see them before they see you bows/magic is the best way, otherwise just chase them down, lock-on, and swing away as fast as you can.

First, they are more of a lizard type if you look closely.

Secondly, you can kill them and you should as they drop a good amount of ores/stones.

They do have a duration they stay there for however, it will run away and disappear after a few seconds so kill it quickly.

yah you can log on to then, if you come close enouth to log on. I'd advise to take a bow and kill them with arrows.
You can kill them multiple times they have a kind of counter, the counter drops each time you kill one or one escape an raise if you kill a boss.
After you killed one (or let one escape) of this lizards take a good place where you can attake it (but not to close to them) then save (by hiting the start button, or so) and load your saved game again, you will start again at the same place you saved and the lizard is hear again if the counter is not at zero.