I frequently find that during one of my campaigns, with a specific group of players, the PCs get very attached to their mounts and very frustrated if they die in combat or become too weak to take along (especially horses and mastiffs).
We've done a few different campaigns, and it's evident that the PCs always want to stick with one mount for the whole game, from Lv.1 to Lv.20 and the end of the adventure. The best situation I ever had was when one PC stuck with a small bird for the whole game and didn't have a mount – however, it did disadvantage them throughout the game.
Are there any mounts or companion animals that are obtainable (or feasible) at 1st level, continue to be good mounts until 20th level, and have a balanced leveling system (so that they don't become too OP too quick or stay weak as the PCs level up)?
Best Answer
How I kept my mounts alive at higher levels
I used two feats for my vHuman Paladin, Oath of Watchers.
Being a paladin, my saving throw bonus aura began at level 6 and was also helpful in reducing damage to my mount. I preferred to bump Charisma with ASIs so that the saving throw bonus helped my mount and all others more; I limited the number of feats I took. Other classes won't have this exceptionally useful aura feature.
Gift of the Metallic Dragon
A feat I have used on other characters could be helpful: the Gift of the Metallic Dragon. (If your table uses options from Fizban's Treasury of Dragons). Proficiency bonus times per long rest you can use a reaction to increase the mount's armor class by your proficiency bonus. You also get one cure wounds spell per long rest, which can heal a mount.
Work with the DM on this one
My DM was great about it. He agreed to exercise the option from the Basic Rules for my mounts: if they went to 0 HP they would go into death saves rather than just be dead. This gave me a chance to either lay on hands or use a healing spell to get them back up, or for one of my party mates to do something similar.
When our campaign ended at level 17, my Griffon had dropped to 0 HP twice. My horses had only been lost a few times, usually they recovered or weren't dropped to 0 HP. Some breath weapons just overwhelm the HP.
Note that this answer works for a paladin. A summoned steed won't come on-line until level 5. For any character, the two feats I mentioned up front will help keep one's mount alive.
Barding
As you go up in level, you'll earn enough gold to buy your mount barding. If keeping your mount alive is important to you, make the investment. Prices are listed in the PHB. For example, plate barding is 4x plate armor, so it's 6000 GP. (And stealth becomes tough to maintain).
How to have a robust mount in Tier 4 (Levels 17-20).
How my Bard, College of Lore, kept her mount alive in Tier 4. OK, this is a little bit of "cheese" but here is what I proposed to the DM.
The DM let me do that for just the hour that I was concentrating on the spell, and then told me "it is now your simulacrum" since he had a world building thing about souls in dragons. Where did the soul for my dragon come from? My bard can't create souls.
Would your DM put up with it lasting until it is dispelled? Maybe.
If we take the rules literally, this will work.
As a counter, an enemy has to figure out that they need to cast a level 9 Dispel Magic to dispel that dragon, or, to make an ability check DC 19 with a lower level casting of Dispel Magic - if we look at how the current version of True Polymorph works - to change your mount back into your simulacrum. In the original version of the spell, first printing, permanent means permanent.
Once per day, for an hour, I got a very survivable mount.
A less powerful version of this is:
Caveat: you probably have to work with your DM on that. Mine vetoed this idea, since he didn't want to deal with the whole party flying around on young silver dragons and he did not like the problem I had created: where would we get dragon souls for each new silver dragon? None of them had hatched from an egg. I completely understood (being a DM myself) as, beyond the world building issue, it would have also put additional burden on him as regards encounter design.
(This most excellent DM occasionally posts here under user name BenjaminTHall)