What's a custom item? How do you make them? And what about custom pet things? How do you make those?

custom items were custom colors avatar items made by players that could be bough for diamond/real mony, nowday this is no longer somthing that is avalable but if you search log enough there is a very small chance to be able to buy one from older players (dont quote me on that, they may have been acount locked by now).

I am unsure what you mean by "custom pets thingy" however, are you talking about re-dye, projects or costumes ?

If you mean being able to create a pet with your own colors and select your own traits, you need an essence to do that.

There are three different ones, regular, enhanced and mystical (I think those are the names), where regular can be obtained through nurturing or battling (rarely), but also I believe have the biggest color swing and you can only give them 1 visible or 2 carried.

enhanced has less color swing and can give 2 visible or 4 carried (I think) but I believe it costs diamonds (think there are certain activities that can give you these but I don't know, I think I ended up with a couple, but I know I didn't buy them. However, it could have been fest activities that gave them to me).

Mystical (or whatever it is called, the best one) has the least color swing, and I am not sure on the number of trait points, I *think* it is the same as enhanced, but I don't know. However, this is, I am pretty sure, limited to the diamond shop. I don't think you can get these outside of there or very rare events.

But, I haven't bothered with essences in a while, so I could be completely wrong :P

If you do mean 'how to create a custom pet via the generator' then it is with essences, guide here (includes details like 'how to use them' and 'where to get them')

(@Jemadar, yes, Enhanced give 4tp, and are found the same ways Regulars are, which is probably where your mystery ones came from- it's just Mysticals that are only from the Diamond shop or as Forum game/contest prizes. Mysticals also do have the same TP count as Enhanced, but with the least colour swing of the three.)

Thanks, I thought that was how they worked, but I get confused with all the items that have different restrictions on them and work differently etc..

If by custom pet things you mean custom pet costumes, I believe those have had the same restrictions as custom avatar items.
There haven't been a lot of custom pet costumes created; I believe they're all the same one or two outfits, just recolored.
As with the avatar items, though, you'd have to find and old player selling them to get your hands on one.