UO Second Age

I was able to provoke the two "invisible" alligators onto the serpent but clearly they aren't able to attack each other. Could this be also related to the bug I've been noticed where when gating into a house while dead, one gets stuck "underneath" the house/tower/etc and cannot move?

when you go "underneath" your house just hit Alt + p to openpaper doll which resyncs your client or have hot key in razor to resync client

I commonly get issues where all types of mobs/players go invisible. The only way I can fix this and make the player/mob become visible again is to open my paperdoll (resync) WHILE moving. I can be in the middle of a duel/fight with someone and suddenly they disappear - still attacking me like normal, just not visible on my screen.

These aligators were not invisible, but they were not on the grass there either. You may with to investigate the area further, so seek the advice of another player for a more complete spoiler.