Their clitorises enlarge to a big dick-shaped schlong, which they use to pee out of, can make erect on command, and even GIVE BIRTH OUT OF. Yes. More on that later. It's hard to tell apart males and females in the wild—this is called genital mimcry. It does come at a price.
It's really hard for hyenas to have sex with each other. They basically have to kind of rub their two dicks together, and hope the sperm shoots up the lady's dickhole. It takes a lot of practice, and it makes females essentially rape-proof, as they have to sort of maneuver to get all the parts lined up correctly.
They also have to give birth through a pee-hole-sized opening, which is almost-certainly incredibly painful, and dangerous to both mother and child. The baby tears its way out of the birth canal, and often mother hyenas bleed to death after childbirth. Sometimes the pups smother, trying to get down the long, narrow passageway. It seems kind of terrible, actually, but we continue to get more hyenas so I guess it works?
Hyenas also pop boners as a sign of submission. Female hyenas rule the pack, so a male greeting a female will often stiffen up in greeting, so she can smell his junk. Even females can harden their "peniform clitorises" to say hello to a higher-ranking gal. Just on command! It's got nothing to do with arousal, apparently, it's just they hyena version of kneeling before royalty.
The world of hyena sex is sort of terrifying, but also sort of amazing. If only we could all show our boners to the people we respect!
Live Science, The Painful Realities of Hyena Sex
Hyaenidae.org, Spotted Hyena: Reproduction