some of the ones i play are:
Dawn of eternity; this one's honestly my personal favorite. i've went on about it before, but the models are just so unique to the game and the staff are really nice people. whatever you mean by customizable, it doesn't have that many manes or markings and such but you can get your presets uploaded to the game by mods. presets are also extremely easy to make on there (in my opinion). I'm probably just really biased but this game, i can not stress it enough, needs a lot more attention. it's also the sister game to last moon but even better.
Dragon's Den; not sure what positive things i got out of this one, but they do have dragon player models and unique looking maps.
Reflection of Spirits; despite the server occasionally being offline, they do have a variety of markings and manes to choose from as well as a leveling system but i'm not sure what it does other than show a number. the models are pretty nice too.
Cereal soup; has lots of markings for different species and has a lot of unique features. the graphics are amazing depending on the settings you use for them, but the controls are extremely wonky if you're new to it.