Interesting reading. 
Ekarus will obviously mutate into an Arachnid. Just look at his avatar and sig banner: don’t those just scream “I wanna be a giant spider”? /yeah, I know, LIZARD DAMMIT 
Thresholds 101: If you inject a serum that’s the same category as the one you’ve been having lots of very specific dreams about, you may end up locking yourself permanently into that category. Most of the intended special-effects haven’t yet made it in. I may PR some before leaving for the family op this weekend.
Fey: Unstable analogue of Alpha. Sexy but high-risk. In the latest version, can go for over 24 hours straight and not even get Tired, though!
Chimera: Ultimate Chaos-Beast, with more mutations planned to make it marginally survivable post-Threshold.
Ursine: Boring Beast-variant with a few more mutations planned, and notably observed once as “an excuse to stick the Hibernation mutation in”, IIRC.
So Chimera and Ursine are both melee units, Fey and Chimera are both high-end and high-risk categories.