The death animation for dying in lava shows the Slayer giving a thumbs up before fully sinking into the lava. This is a reference to the ending of the 1991 film "Terminator
2: Judgement Day". This animation also appears in Doom Eternal
In Level 6:Kadingir Sanctum in the cave where you get the Chaingun there will be a head of a skeleton on a spike wearing a football helmet. The football helmet itself looks very similar to the one Commander Keen
wears. Both Doom
and Commander Keen
were made by ID Software
The Cyberdemon's theme has hidden pentagrams and 666's that can be viewed in the song's spectrograph.
The trophy/achievement "Shoot it Until it Dies" earned after defeating the Cyberdemon boss is a reference to an infamous "pro tip" once created by Doomworld co-founder Andrew "Linguica" Stine as an April Fool's joke that over the years has been mistaken for a genuine scan from an old GamePro magazine.
Images leaked from the time the game was still being called Doom 4 show a completely different looking game. The setting seems to have been post-apocalyptic, and some images show soldiers in a broken down city. Not much else is known about this early version.
The Cacodemons went through a redesign during the development of the game. They were recolored and slightly altered to look more like their original sprites.