Behold! The revival of Xen Infection! This time, Mo' Xen Fauna and Xen Infection have been merged into one addon and given a whole new look! (except for the houndeye, but we don't have to talk about that)
REMEMBER TO ENABLE ALL EXPERIMENTAL GAMEPLAY OPTIONS OR SOME FEATURES MAY NOT WORK!
Creatures
Antlions

These fellas are quick! They spawn in groups, however are relatively easy to kill.
Antlion Showcase
Houndeyes

These cute peeper puppies are small, but can be dangerous! If you see them clustered up, there's a leader houndeye nearby! Leader houndeyes make them collect into groups and attack whoever the leader sees. Similarly to the original game, the more houndeyes there are in one area, the stronger their attacks get!
Barnacles

Barnacles will spawn in caves and dangle their tongue from the roof. Keep an eye out for it when mining, or you might get grabbed! It stores most of the items it eats (including your loot) and you can recover everything should you find the barnacle again after you died to it.
Barnacle Showcase
Headcrabs

What I'm sure plenty of you have been waiting for! The headcrab is a parasitoid that latches onto your head, should you let it weaken you enough. It only latches onto humanoids (illagers, villagers, players, etc), as their biology is the most compatible with their parasitism.
Headcrab Zombies

The next stage in the headcrab life cycle! Anything that the headcrab successfully latches onto will become a headcrab zombie. These fellas have long, mutated arms that can reach far! Be sure to land quick hits on this guy before it eats anything, because it can gain lots of health from its evolutionary stages, where it gradually evolves into the next stage-
The Gonome

The gonome is a large, beast-like stage of the headcrab zombie. If a zombie eats enough, it evolves into this brute! This fella can smash walls, deal massive damage, and run very quickly! You better come prepared if you wanna fight this guy...
Other Stuff
The Gonarch

The last stage of evolution for the headcrab. This currently hasn't been fully implemented, thus it does not spawn in survival. It does, however, have semi-complete behaviors should you want to spawn it with creative/commands!
Details
This addon is in early stages, and such is succeptible to lots of bugs. Should you encounter any of these bugs, feel free to head over to my Discord server to report them!
Check out the Discord here!
Should you upload this to a 3rd party, please credit me for the work. I understand now that some people can't access CF addons on their device, thus have to resort to certain apps.