Today while I was killing 4 a quest 2 monsters gave me problems with their HP.

One was the Tree Spirit Guard i think their called. One of them had no HP at all but just stood there and I couldn't attack.

The other was the Rough Tail Fox. When I attacked some of them they would lose their HP but instantly regained their HP and didn't chase me.

My characters name is _Soda, and I'm in Gaia on the Light side.

The monsters that just stand there with no hp are already dead and it's a glitch that hasn't been fixed.

I don't know about the fox but monsters do regain hp slowly if you don't attack them for a while.

yes but the fox I kept attacking and it healed instantly. It's name was green, the HP disappeared, but then it instantly healed and didn't come after me.

TheFatKat has already covered the no HP glitched mobs - they have been there since I started

The other issue you mention is a lot more rare than the glitched mobs one, but it has also been around for as long as I can remember.
It seems to be that there are certain spots where the mob can't "see" you.
You hit it and get aggro, but as the mob can't see you it automatically resets & goes back to full health.
The solution to this one is to move away, then come back at a different angle and try again.
If that doesn't work, just try a different mob

Alright then Thank u for the information xD i really appreciate it