Mark of Handling
The Mark of Handling gives its bearer a primal connection to beasts and the natural world, granting the power to calm and coax. This extends beyond purely natural animals; the mark allows its bearer to guide a hippogriff as easily as a horse.
Wild Intuition. Whenever you make a Wisdom (Animal Handling) or a Intelligence (Nature) check, you can roll a d4 and add the number rolled to the total ability check.
The Bigger They Are. Starting at 3rd level, you can target a Beast or a Monstrosity when you cast Animal Friendship or Speak With Animals, provided that the creature's intelligence is 3 or lower.
I apologize for starting off with a block quote, but a search of previously asked questions didn't provide links to anything immediately obvious that suggested this subrace in particular has even been addressed here, so if I'm going to seek a subjective answer, I thought it best to open up with context and demonstrated research, and in this case that requires citation.
So this is a pretty deep well. After thoroughly reading these (related) questions, and the associated answers, one particular comment by Darth Pseudonym stands out to me as particularly insightful and relevant here.
The skill uses given in the book are not the be-all and end-all of what the skill can do; they're "examples of how to use" skills (PHB p.174). It's improper to claim a skill can't do something if it isn't listed in the book and therefore a 20 on the roll does nothing, or should not be allowed. Rather, say the book does not specify the results if a DM allows a non-standard skill usage, which means it's up to the DM to determine how to adjudicate it.
I agree wholeheartedly with this sentiment, but I'm sitting in the player's seat for this one.
The idea is a Mark of Handling Moon druid who has a personality trait of preferring the company of wild creatures to most people- "People are dishonest and hide their nature, while wild creatures embrace it for all the world to see," and a flaw of "All the wild creatures I meet are potential allies- ALL OF THEM."
I'm not planning to go out and track down the Tarrasque or something blatantly game-breaking and insane like that. However, I find it entirely plausible that I may at some point come across things like griffons, hippogriffs, pegasi, unicorns, owlbears, worgs, gorgons, rust monsters, cockatrices, or even rocs.
All of these creatures fit the classical archetype of 'wild animals' with the exception of the fact that they're magical fantasy creatures – 'monstrosities.' Numerous of these creatures would make welcome companions to the party. The druid in question is invested heavily in both wisdom and charisma, and so to speak, would like to eventually 'catch them all,' in-so-far as the opportunity presents itself without derailing from the actual mission at the table.
Because this is an off-text application of related skills, and because there's as much a social component as a 'handling' component when approached this way, the druid is investing ASI levels into both wisdom and charisma through multiple skill expertise feats, gaining expertise in animal handling, athletics, deception, intimidation, insight, perception, and persuasion (5 Skill Expertise + 1 rogue level at level 1.) This ends with 20 wisdom and 18 charisma.
The point of all this is that I've gone for covering the bases of the most likely type of 'social' interaction with monstrosities with expertise, from racing and wrestling, to posturing and growling, dealing with them honestly or telling them a lie (come hunt with us, we find the tastiest meats/we didn't steal your egg, it was that harpy that just flew over the mountain behind you). My approach here is limited only by the situation and my creativity.
Please, Fallen, my eyes are bleeding; what exactly are you asking?
What I'm looking for is suggestions on ways I can make this fun for the other people at the table with me. There's an easy-bake happy button for the party in being able to potentially give tame flying/fantastic mounts/guard pets for them- and the idea sounds fun in my head. But I don't want my pokemon stable to dominate the entire play session, and I don't want to make the DM's job so difficult they have to give consideration to just cancelling or re-rolling any random encounters involving wild monstrosities.
Are there any established rules or costs to taming/training such creatures? If not, has anyone come up with a system of checks utilizing downtime costs or other measures to keep this from becoming a major problem on either side of the table? If so, how did it work for you?
Best Answer
Have you talked to your DM about this?
The short of this is: there are no explicit player facing rules for befriending, taming and training wild animals as ongoing companions (and neither for monstrosities) in 5e. So anything you want to do here will require the buy-in of your DM, and you should discuss your character concept with them.
There are also no general downtime rules for taming and training animals. That said, the rules for mounts state:
In line with the rules earlier editions had about this, taming a monstrosity to act as your mount or companion should be a long and involved process. The dragonmark may allow you to guide them as a mount for yourself, but commanding them to do anything else still will need training.
Some entries in the monster manual, such as the griffon tell you explicitly what it takes to tame them:
That suggests that if your DM allows you to do so yourself, it should be difficult and time consuming to gather even one, never mind multiple such companions. Further supporting this is that even classes that are specialized in animals companions, such as the Beastmaster ranger, typically get a single companion only. For a druid, a Shepard would seem like a more natural choice than a Moon druid who specializes in self-transformation.
Neither Speak with Animals nor Animal Friendship indicate they have a long term effect. The former states you might be able to convince the animal to do you "a small favor". Animal Friendship has a 24 hour duration, and from your comments I understand you plan to maintain charmed creatures by re-casting it daily on them. This would work, although it could consume a good part of your available spell slots depending on how large you want the "stable" of creatures you have access to to be. You also need to get in touch with all of them every day, which may or may not be easy depending on the campaign. You would need to use Speak with Animals to communicate anything that they are not trained for. If you treat them well, over time they could become naturally friendly with you, and not require magical aid any more -- the details of this would again be up to agree on with your DM.
Lastly, there are several posts on this site that discuss the playability problems caused by having a stable of minions tagging along. Aside from balance questions, the conclusion is that it mostly is not a great idea to do it, and if you do it you need ways to work around the issues it causes. It might be useful to consider these with your DM, too. Limiting it to one or two creatures per session is probably a good idea.