Meeting certain conditions allows to become some sort of monstrosity known as a Slayer

Link is able to purchase and wear various masks that transform him for various purposes.

Alucard has a few animalistic forms, namely a wolf and a bat.

Atreus has his Spartan Rage equivalent turn him into a wolf.

Similar to Tekken 7, Kazuya is able to transform into Devil Kazuya, this time by going into Heat Mode

Dante is back to just one form of DT this time, although his appearance changes slightly depending on the weapon equipped. His brother Vergil can also access DT in his boss fights, as well as when he was made a playable character in the special edition re-release.

Dante gets greedy again and has two DT forms like in DMC2, only this time the 2nd form is far less obtuse and is actually useful. Nero gains a proper full body transformation once he acquires DT, but this happens in the final mission so you won't really get to use it to its full extent until subsequent playthroughs. V has a DT but it simply changes his hair colour, the main attraction of the mode is bringing in his strongest summon, Nightmare. Once again, Vergil was added in the Special Edition release.

Link is transformed into a wolf. Eventually he is able to morph between human and wolf form at will.

Druids are able to transform into a werewolf and werebear.

Ditto. Rodin is playable in Tag Climax and has a more traditional Devil Trigger style transformation.

Sol Badguy has his Dragon Install that powers him up at the cost of decreasing his life over time.

Clive has access to a Limit Break that acts like a traditional DT mode, changing his appearance and granting him regenerating health and extra damage

Ditto.

Both Samanosuke and Jacques can tap into Onimusha form, this time it being a manual activation.

Bayo (and Jeanne) are able to morph into various animals, mostly for traversal purposes.

Your character is part-Yokai in this instalment, meaning they can tap into an ability called Yokai Shift to temporarily become one of three Yokai forms that change depending on what guardian spirit you have equipped.

Dragon Install has been moved to Ky Kiske for this release.

Depending on what questlines and guilds you choose, you can become either a Werewolf or a Vampire Lord.

Ditto.

Kazuya is able to go into his Devil form if he spends his Rage Drive

You're a kid now, you're a squid now.

Ditto.

Jubei can transform into his Onimusha form after collecting 5 purple orbs. Interestingly, this transformation triggers instantly and cannot be toggled manually.

A weird case here with the Summoner class added in one of the DLCs, which lets you "summon" Bahamut, but really in gameplay Jack just seems to transform into him

The Prince gains his Dark Prince form in this game. Maybe it was from listening to too much Godsmack during Warrior Within.

Dante keeps his patented DT transformation. New protagonist Nero has a DT of his own, but his body doesn't transform so much as he gains a JoJo-esque stand. Special Edition would once again add Vergil with his DT.

You get the idea.

Bayo has swapped out her animal transformations for something called Demon Masquerade, wherein she can merge with the demon she has equipped based on the current weapon. Newcomer Viola has a more traditional DT style mode where she transforms into her Faerie form.

The first instance that I personally remember of this, Dante has access to a Devil Trigger mode wherein he taps into his demonic blood to temporarily become a demon.

Travis can become a tiger because....reasons.

Ditto.

Protagonist Jen can transform into different forms with various different uses.

Ditto.

War has his Chaos Form, granting him complete invulnerability and a boost in damage.

Ditto for Death's Reaper Form.

Your character has a transformation but from a story perspective, it's only happening in his mind.

A fighting game series where the main hook is each character being able to transform into an animalistic version of themselves? Yes, this definitely counts.

Ditto.

Ditto for Fury's Havoc Form.

I believe you are able to transform into some alien mutant thing partway through this game.

Being a MOBA for Blizzard's various IPs, you get to play as some of their more iconic characters, a few of which have transformation states including Illidan's Demon Form, Greymane's Worgen Form and Alexstraza's Dragon Form

Same goes for this modern reboot.

Perhaps one of the first examples, you are able to morph into various forms of animals.

Dante actually has access to two DT forms in this one, although since the method of getting the stronger one involved being on low health in a game this easy, not many people got to use it. Also the deuteragonist, Lucia, has a DT equivalent .