Going to have to dig up some old design notes for this one.
spyro
easy, obvious first answer. it'd be weird if he wasn't on the list. a lot of those early design decisions were either thinking about how things would be the same or different (for example, dex has similar restrictions in terms of gliding without powerups).
surprisingly not the reason for the purple scales though (particularly after seeing a lot of barely changed OCs), although i'm sure a subconscious factor, which leads me to...
saints row 2
at some point prior to 2009, red was our favourite colour.
after saints row 2? it's been purple ever since, a mandatory part of character designs.
the artic blue gba/purple gamecube combination
after choosing purple as being a main colour, i went on some colour palette site to find one that would work well. while not intentional, i suspect i was subconsciously influenced by this pairing, which i view as the canonical colours for those consoles (and also the ones i had) - particularly with my equivalent to the gamecube's handle
yooka-laylee (pre-release)
one of those early interviews about yooka-laylee near the launch of the kickstarter talked about yooka's abilities as a chameleon, how form influenced function and vice versa.
it's obvious in retrospect, but i'm pretty sure this provided a lens to look back at things that had previously just subconsciously been accepted; what makes sense and what doesn't about other platformer characters. dex was designed around aptophilia before that word had been coined.
dynamite headdy
also spyro again, jurassic park: trespasser, dead space
the heart tail happened because i wanted a diegetic health indicator, some way of showing things within the world. i always really liked dynamite headdy's spotlight system for this, although it barely qualifies as diegetic - so my tail is basically a single coloured version of that
(there's one comm that shows off the trespasser influence, as well)
bubsy
also banjo, crash
a lot of backstory thoughts (at least initially) were just old headcanons about bubsy, reassigned. what did he do after bubsy 3d? did he sit around in a video game character retirement home living off a pension and meagre residuals? did he have to get an actual job?
to a lesser extent, this same process happened with banjo and crash, too, some fangame ideas that got incorporated into what dex's games would have been.
i wonder what would have happened if all three hadn't been in an extreme period of franchise drought around 2015
gex
it is and always has been tail time. surprisingly little (intentional) name influence though
sunset overdrive
also ratchet & clank and probably others
i've given the actual explanation for dex's horns elsewhere, but sunset overdrive's ability to grind underneath rails (as well as games that featured ziplines) helped provide a logical justification
yoshi's island
also banjo/yooka, action replay, cheat codes in general
this didn't factor into initial design, but i've ended up getting a lot of TF art - which i consider all part of yoshi's island style transformations; a temporary powerup to accomplish a specific task; minimal risk of turning into a washing machine
and for a lot of other things, some of those i've considered π being a bad influence, but some of it is just dex being incredibly susceptible to cheats being used
closing summary
a friend once said that dex felt like a character only we could have designed
and looking over this list... yeah
not just the things we played but when we played or thought about them were all pretty important
oh, also, forgot one
my little pony - friendship is magic
pinkie has never been our favourite pony, but this GIF has lived in our head for a long time rent free
things are just more fun with the fourth wall being easy to pierce
sometimes you need to remember the vast series of dice rolls that got you where you are.
remove any element, and the formula changes
