A lion, a big adult male lion.
Let me explain. I used to work in Africa, doing fieldwork in large carnivore conservation and research. After doing that for several years I noticed my condition detoriated. When a 10K fun run was organized I enthusiastically started the run, as if I was still capable of running a 38min 10K like I used to do. It was humbling, I had to walk stretches and couldn't finish under an hour (ok 35+ Celsius and a nearly 11k 10k didn't help, but still, humbling).
So I decided to start exercising again. First running in the off season, doing HIIT (somebody broght some exercises of Shaun T - "Keep the core contracted!" to the camp etc). I got shin splints, tore and achilles, but made progress.
A few years later I was working in a National Park which was very open and our main focus wer African wild dogs (if you've seen the wild dogs in the BBC Series The Hunt, that were 'our' wild dogs), cheetahs and hyaenas, but also the single pride of lions in the park. Most work we did with motorbikes, but were always careful to not run into the lions with our bikes. We did have an airstrip near camp, which was good for running. The consisted of loose sand, which was compacted at the airstrip, the airstrip was fenced, had markers every 100m and was 1km long, which all made for good running.
One day I went for a run in the afternoon, starting a little later than anticipated, but since there was a large crew working on the airstrip I wasn't too worried and walked to the airstrip (about half a mile from camp), and didn't take a motorbike or car. During my training everybody left, and when I finished I was alone and it was starting getting a little too late to comfortably walk around. The area between the airstrip and camp was just grass, while camp was located in a little woodland.
As it was late I jogged back rather than walking, but when I was about half-way, the adult male lion of the pride jumped up out of the grass and charged. So I stopped. I've met lions on foot several times. Stand your ground, make yourself big and make noise and they'll back off. I could hear the generator running in camp, so I realized that my collaegues wouldn't be able to hear me shouting and clapping. The lion stopped his charge at about 20-25m, so not very close (I have experienced them stopping much closer), but after stopping he didn't turn around and run off (as usual). He just stood there, growling and swiping with his tail. This stand off probably last much shorter than I remember it, but at some point I decided I should try to get some distance between him and me and slowly started backing off. This worked initially, but when the gap grew to about 80m he started following. Not good! And I still had to walk a few hunderd meters until the edge of the woodland, not good either!
I continued, and he continued following and at some point there was a little bush, which I thought, if I manouver in such a way that that becomes between us, it would break the line of sight and he might relax. I have to say, that this bush was not something I could climb for safety, it was less than 2m high. It didn't work, he kept following.
I got closer to the woodland, and I was hoping he would give up. The male lion never really ventured into the woodland where the camps were (tourist camp, ranger camp and research camp), but the females often did. The females never showed any aggression though and sometimes they would lie in the sand watching you from 10m where you were cooking. After shining a torch on them they would sometimes just roll on their backs almost behaving like domestic cats (almost, half an hour later they would kill and eat a wildebeest). A few documentaries about these lions were made, The Last Lioness being the most famous one, I think you can find it on vimeo and possibly other platforms.
Anyway, as we were approaching the woodland, I though he would give up. The distance between us had increased a little, maybe to 100m now, and I was about 15m from the edge of the woodland. However, he started jogging! So I started shouting and whistling as hard as I could. And because I was close to camp now, my colleagues, who had just been wondering why I was still not back, heard that and thought something might be amiss. And then things happened rapidly. He let out a loud road and broke into a full sprint. My colleagues now realized the problem! And I, knowing that you should never run from a lion, but since he had already been following me for several hunderd meters, decided I should run, and I chose to run to the closest camp, the ranger camp, and dove into an open tent there, startling the rangers.
One colleague had jumped up out of his chair and ran straigth TO the roar. When he ran full speed around the corner of the driveway he saw the lion and as he ran at the lion the lion stopped and turned and ran off (side note, my colleague is the 'colleague' in this article, who at a previous job saved a colleague out of the mouth of a lion:
https://www.dailymail.co.uk/news/article-2056680/Kgalagadi-ranger-Albert-Bojone-dragged-lions-MOUTH-brave-colleague.html).
We all had a good laugh and a few beers shortly after this! And from then on I didn't run as late in the day anymore, always took a car, and always checked with radio tracking gear if the lions weren't around. It turned out that the lioness and their cubs were on the other side of camp. Thus the camp, and I, were between him and the lioness and cubs, that could have played a role in his behaviour.
The next day we checked his tracks and he had been sprinting flat out (huge gaps between tracks), with his claws extended:
The trouble maker:
Not too worried about the wildlife I encounter on my runs in the UK these days!
