The catoblepas appeared in various monster manuals throughout D&D's editions. Have WotC published stats for it in a D&D 5th edition product yet? If so, which?
[RPG] Does a Catoblepas statblock appear in an official D&D 5e product
dnd-5emonsters
Related Solutions
It appears that you cannot.
The issue here is not copyright. Titles and short phrases generally aren't protected by copyright — there is trademark law for that. Dungeons & Dragons certainly is a trademark, but even that isn't really the point here — you can probably use trademarks in a nominative sense even without special permission (hello, Boston Marathon). The important thing is: you want to use 5E stuff beyond just the name, and the OGL — the Open Game License — is what lets you do that. If you want use that license, you have to do what it says. The relevant section is:
- Use of Product Identity: You agree not to use any Product Identity, including as an indication as to compatibility, except as expressly licensed in another, independent Agreement with the owner of each element of that Product Identity. You agree not to indicate compatibility or co-adaptability with any Trademark or Registered Trademark in conjunction with a work containing Open Game Content except as expressly licensed in another, independent Agreement with the owner of such Trademark or Registered Trade mark. The use of any Product Identity in Open Game Content does not constitute a challenge to the ownership of that Product Identity. The owner of any Product Identity used in Open Game Content shall retain all rights, title and interest in and to that Product Identity.
... and in the 5E SRD, the phrase "Dungeons & Dragons" is clearly designated as "Product Identity" — in fact, it's the first thing listed. So, unless you have a separate agreement, it looks like the answer is no — but, of course, keep in mind that I am not a lawyer, and this is just the interpretation of some guy on the Internet.
Note that the wording specifically forbids use of this "Product Identity" — including "Dungeons & Dragons" — in conjunction with a work, not just "in the work itself". This would appear to cover the website situation. This is probably more restrictive than normal trademark rules — but that's basically part of the price you pay in order to get the benefits the OGL grants you.
As with "Marathon Monday" or "The Big Game" in the sports world, you'll probably need to find an alternate phrase which your audience will understand to indicate compatibility without violating the license.
Sources where they first became available to players:
PHB = Player's Handbook
MM = Monsters Manual
MToF = Mordenkainen's Tome of Foes
SCAG = Swords Coast Adventurer's Guide
VGtM = Volo's Guide to Monsters
GGtR = Guildmasters' Guide to Ravnica
WGtE = Wayfinder's Guide to Eberron
DMG = Dungeon Master's Guide
EGtW = Explorer's Guide to Wildemount
Standard Languages:
- Common (PHB)
- Dwarvish (PHB)
- Elvish (PHB)
- Giant (PHB)
- Gnomish (PHB)
- Goblin (PHB)
- Halfling (PHB)
- Orc (PHB)
Standard Languages (Ravnica):
- Abyssal (PHB)
- Celestial (PHB)
- Common (PHB)
- Draconic (PHB)
- Elvish (PHB)
- Giant (PHB)
- Goblin (PHB)
- Kraul (GGtR)
- Loxodon (GGtR)
- Merfolk (GGtR)
- Minotaur (GGtR)
- Sphinx (MM)
- Sylvan (PHB)
- Vedalken (GGtR)
Exotic Languages:
- Abyssal (PHB)
- Celestial (PHB)
- Draconic (PHB)
- Deep Speech (PHB)
- Infernal (PHB)
- Primodial (PHB)
- Aquan
- Auran
- Ignan
- Terran
- Sylvan (PHB)
- Undercommon (PHB)
Race/Class-Specific Languages:
- Aarakocra (MM)
- Druidic (PHB)
- Gith (MToF)
- Thieves' Cant (PHB)
Forgotten Realms Human Languages (requires GM permission):
(All in SCAG)
- Dambrathan
- Bedine
- Alzhedo
- Chondathan
- Damaran
- Waelan
- Guran
- Halruaan
- Illuskan
- Roushoum
- Chessentan
- Mulhorandi
- Untheric
- Thayan
- Rashemi
- Shaaran
- Shou
- Tuigan
- Turmic
- Uluik
Eberron's Common Languages
- Common (PHB)
- Dwarvish (PHB)
- Elvish (PHB)
- Giant (PHB)
- Goblin (PHB)
- Orc (dead language, PHB)
- Infernal (PHB)
- Riedran (WGtE)
- Quori (WGtE)
Monstrous Languages (available via Favored Enemy or Training):
- Blink Dog (MM)
- Bullywug (MM)
- Giant Eagle (MM)
- Giant Elk (MM)
- Giant Owl (MM)
- Gnoll (MM)
- Grell (MM)
- Grung (VGtM)
- Hook Horror (MM)
- Kruthik (MToF)
- Modron (MM)
- Otyugh (MM)
- Sahuagin (MM)
- Slaad (MM)
- Sphinx (MM)
- Thri-kreen (MM)
- Tlincalli (VGtM)
- Troglodyte (MM)
- Umber Hulk (MM)
- Vegepygmy (VGtM)
- Winter Wolf (MM)
- Worg (MM)
- Yeti (MM)
Book of Vile Darkness
- Dark Speech (DMG)
Wildemount's common languages
- Zemnian (EGtW)
- Marquesian (EGtW)
- Naush (EGtW)
Best Answer
Yes.
The catoblepas was published in Volo's Guide to Monsters (page 129) for 5e. It is available here on D&D Beyond behind a paywall that requires purchasing the content.
Note that you can also use D&D Beyond to search monsters that are available in official D&D publications. You will be able to see the search results including the monster's name, type, CR, and publication source even if the full stat block is behind a paywall. (For example, here are the search results for "catoblepas.")